I decided earlier today to play around a bit with a hex editor and hack a copy of my Bard's Tale 1 save. Here are some of the things I was able to do:

* Class change. Casters retain their spells, but fighter-types with spells can't cast them. Archmage, Chronomancer, and Geomancer can be accessed in BT1 this way.
* Interestingly enough, if you load a save where a character's XP is below the minimum required for their current level, it gets increased to the minimum. (This is probably done because of a patch that increased non-legacy XP requirements at high level; it also helps in the case of a legacy XP BT2->BT3 transfer, where fighter-types would otherwise end up with not enough XP for their level.
* Learning level 1 spells in your new class costs 0 gold. Archmage, Chronomancer, and Geomancer spells can be learned in BT1. This includes the spells to warp you to and from Arboria, though (I assume) there's no place to use the spell in BT1 or BT2.
* I was able to hack bard songs as well (they're stored as a bitfield). This gives me access to the BT2 and BT3 bard songs in BT1. Yes, this includes Kiel's Overture, and allows me to roast 396 Berserkers with that bard song. (If you're worried about game balance, the only thing that's obviously unbalanced is playing Kiel's Overture during combat, and even that is fair if you're in BT2's last dungeon because the enemies have so many HP.)
* Brigaround Ballad, a bard song that is normally only available in BT3 if you choose the legacy bard songs option, can be hacked onto your bard in the other games. The song functions properly, using its BT3 tune, but it works exactly like a certain BT1 song from a game mechanics standpoint. Unfortunately, there appears to be no way to get the BT3 versions of the songs it shares with BT2 without enabling legacy mode.
* If I can figure out how to hack spells, I want to hack the Dreamspell (from BT2) into one of the other games and attempt to cast it outside of combat; if I understand how things work correctly, this spell should attempt to teleport me to one of the BT2 dungeons. Could this be used to teleport between games without transferring characters, or will the game misbehave?
* When the game encounters invalid data (like if the character's class variable is bigger than the value corresponding to Geomenacer), the game will appear to hang, but pressing F10 will still open the menu, allowing you to open another save (or the same one, modified by another process).

I still haven't figured out a way to hack spells (any pointers?), but I *did* figure out how to change Garth's inventory; just find an item in Garth's inventory with the same length name as the item you want, and change the item's name. Then, when you go to Garth's, the item will be there.

All items from BT3 cost 10,000 gold, no matter how powerful. (It sounds like this would have been the original price of the Wineskin, which would have allowed you to buy one for 5 gold and sell for 5,000, giving an extremely obvious gold exploit.)

I was able to get The Scepter (BT2 endgame item). Here are some observations:
* It cost 70,000 IIRC (might be mission a 0). That was more than half my party's combined money.
* This left me without enough money to identify it at Roscoe's, but I was able to identify it there anyway, despite not having enough money, because of a glitch (that's still present in 3.24). (If I couldn't identify it this way, I could, of course, fix this by changing someone into a Chronomancer and learning WHAT.)
* Nobody can equip it. (It seems this distinguishes between Wizards and Archmages, unlike certain other items. I did not check to see if a Chronomancer or Geomancer could equip it; in classic BT3 they can equip anything an Archmage can without exception (and Geomancers also get Warrior equipment).) However, with the Legacy mode option to use unequipped items, I could still use it as an item without having to equip it. (Is this intentional?)

Is it possible to change the enemy Monster attributes like AC and HD?
I do this with my Pool Of Radiance Game for example and thought it might be fun in this game as well.

If so ,how would I do it?
I have not used a Hex Editor before ,if that is the technique, so maybe guide me to the most user friendly and what files I need to look at?

It is most likely possible, but I do not know how to do it.

The best approach would be to figure out what tools were used to develop the game, and use those tools, or others that understand the same file formats, to do the changes.

Modern games like the remaster just store things differently from how older games like the original trilogy.

I can't figure it out with the Steam version of the Remastered edition, but it was quite possible to do so in the Apple IIGS version using a hex editor. The monster names show up in multiple files.

I recall being annoyed at how useless the TADU "Target Dummy" spell was, and how weak ANSW was in BG1, so I edited the monster data to make them worthwhile, especially considering that they can only be used for a single battle. I gave them max HP so they could take some hits.
The problem is that I did it 30 years ago on my Apple IIGS, I don't recall how I did it and my notes are faded.

Worth noting that the Target Dummy has an interesting property in the remaster; enemies will prefer to attack the dummy over attacking your other party members. I believe the dummy may also appear in the first slot rather than the last one.

The Wizard Wall summon, I believe, also has a similar property in the remaster.

As for editing the remaster save files, I noticed that they have a string like CSharp-Assembly early in the file, which some google search seems to indicate it's some Unity thing, and there's apparently some tool that can be used to examine such files. More research may need to be done, but using such a tool to edit a save file might turn out to be feasible this way.

(As always, when editing a file, make sure to make a back-up of the file first.)

Do you have any idea how to edit the properties or change Legacy mode? Specifically I'd like to be able to sell Fire Horns, Dayblades, and Lightwands to Garth and buy them back later when they aren't common drops. However, none of the ones I get as random drops have full charges and Garth is throwing them away when I leave the shop. If they want to get rid of the recharge "exploit", they should at least make it so that the items drop at full charge. Alternately, store the items individually so that you can buy back with fewer charges. How they did it was clumsy and changes my way of doing things. Until I could find Mage Staffs I relied heavily on light-making items, and would store them at Garth. You can only store 10 items at the bank, so that is NOT a good substitute.

Alternately, how to start new game and still have the contents of Garth's inventory? Alternately, can I hack my game to turn on that legacy mode feature of Garth not throwing away items with partial charges? I'm on the third floor of the catacombs and don't want to start over. I'm not a kid anymore, I don't have all summer to just play games.