Well yes, keeperfx is open source, so anybody can take the source code and do whatever changes he likes. Additionally, one of the most important changes in KeeperFX is making everything configurable, so if you're not a software developer you can still change a lot with a map-editor and a text-editor. That being said, most people will play the release as is, and it is different from the original game. 'Fan Expansion' might be the proper term here.
Most of the work that has gone into KeeperFX, is simply remaking the game in open source, and by one developer doing this in his free time this takes a while. While it is being remade, it has been brought into this century, allowing it to be played on modern machines in high resolutions, and like mentioned things being made configurable and the possibility for custom maps added.
As for the changes, a lot of them are straight up bugfixes, but they of course affect gameplay. The original game was bugged to hell and back, some stats(dexterity) would 'loop' back to 0 causing a lvl10 horned reaper to be almost unable to hit any creatures, other stats would hit a cap (causing a lvl 10 giant to hit no harder than a lvl 6 giant) and spells of lvl 10 creatures where so bugged out that you were better off not training Dragons beyond lvl 9 for example.
However, bullfrog sort of balanced around the bugs instead of fixing them, so after KeeperFX fixed the stats the balance was gone. Suddenly all the creatures needed to be re-balanced. As the developer himself focuses on remaking the game itself, a keeperklan-forum user took on the responsibility on changing/testing all the stats of the creatures, but unfortunately he went a bit beyond the basics and for example gave some creatures some new spells and everything. He realized he went too far, said he would restore the frivolous changes, but disappeared before he finished. This was left for a long time, but in 0.4.6 the balance should be more or less ok, although some of the crazy spells are still there, hopefully they will be restored sometimes. If you think a creature a lot stronger/weaker than they were in the original game, report it and it might get corrected.
There are also several straight up fixes,.. fairies don't get stuck in the ceiling all the time, workshops don't fill up with glitched crates, keepers will defend their hearts when they are attacked, stuff like that.
However, not all 'fixes' are for the better, the 'problem' of flies dying all the time has been fixed by rethinking how fear works, and now when you have 'flee' enabled some creatures flee for no reason. I made
a patch to restore this and the creature spells back to the original.
What does remain though, is that as the keepers now behave a bit differently, some of the original maps also need some further tweaking to play more like the original. This is something that has little priority now, as the remake is still being finished and this is a job better left for when your done making technical changes. On campaign lvl 15 both keepers may now die because they dig towards the heroes too soon for example.
A final upside, is that some
new script commands have been introduced, so that mapmakers can
do stuff not possible with the original game.
All in all, even without the increased resolution and considering some of the questionable changes I think KeeperFX is more fun to play than DK1 was, but if you're looking to play exactly the same game you did 15 years ago, this is not it. Not even if you restore all the stats and play the maps from the original game.