Yo.

All my dudes (more like dudettes) are priests with 18 str and lots of vit for maximum CC and lots of mana reg (+1bard + 1 Mage). So I cannot class change except into Fighters all that quickly.

What I heard is, that I get a miss reduction when I change class at level X, and then level up to X again, whereas I get the bonus for level X again, as long X is the highest level I ever had. First question: Can I see my highest level ever somewhere, or do I have to remember it myself?

And what is the best strategy to make use of this? Class changing asap (not that I can now) seems to mean not having many hitpoints for quite some time. Class changing late seems to take a lot of time. (Does it?). How often should I do this? I a miss chance of 0 necessary, realistic, relevant? And what class should I change into? One with much HP growth? Does class changing my mage into fighter hinder his magic?

Second question is: How does class changing change mana point growth? Does it at all or do the respective skill do this or the spells? When my mage leveled up, his fire stuff increased from 3 to 9. (Did not learn an additional fire spell, but of course I already had one).

1. No, you can't see your highest level ever anywhere
Do not change any character into fighter, all stats except strength will be reduced to the racial minimum so it will be hard to get the stats for the changes into classes you want to end the game with.
Miss chance is not important enough to bother about, you'll hit anyway.

The game was designed and balanced for being played without class changes, I played my game without any and it wasn't difficult, you only cheat yourself away from the real gaming experience and cause a lot of work for yourself by class changing excessively. If you want to change classes once I recommend level 8 or so.
Be aware that you won't get the all the stat increases you need on its own so you're on heavy save/reload duty at level ups.

2. With each level up you also get an increase in mana in pools you already have spell points in. It's only +1 if you don't reach the highest character level. You also get spell picks and skillpoints for raising the casting skills, the mana you get at a normal level up also depends on the number of spells you know in this domain.

Your highest level isn't listed ingame, but you can see it through the Saved Files Viewer of Cosmic Forge.

It's generally best to switch at level 8 or lower. The XP to get from 8->9 is the same as the XP to get from 1->8, so that's one of the better points to do it. But if you're starting as a Thief or something to get access to Ninjutsu, for instance, you might as well switch classes immediately.

Honestly for HP it doesn't really matter all that much, because once you get to level 20, every levelup is at 600,000 XP like clockwork, regardless of class. As long as you aren't a Mage or a Bishop, you'll be swimming in HP by the end of the game. Obviously you'll want to switch to a class with more HP earlier if you want more HP by the end, but it's not crippling if you don't.

Mana regeneration is determined by your Piety and Vitality upon character creation. It never changes throughout the game. VIT and PIE of 14 each gives you a +1 to all of your regens, and 17 of each gives you +2 to all of them. Being a faerie also adds +1, while being a Lizardman gives -1 (to a minimum of 1). If you want the best mana regen possible (which will also give you more spell points every level) you should start as a Mage, Priest, Alchemist, Psionic or Bishop with the highest VIT and PIE you can manage, and then switch over to another class later if desired.

The other important thing is that your hit chance and number of attacks per round only increase up to level 19. If you switch classes after that, they won't increase. So if you want to switch your Mage to a Samurai, for instance, do it before 19 or you'll never get 4x4 attacks.

I assume Senf is asking about Wizardry 6 (else maxing strength won't make sense) and you're using the different W7 rules.

In W6 different classes have different XP requirements for late level ups (225,000-475,000 dependent on class instead of 600,000 in W7) and 16+ vitality at creation grants a bonus to spell regeneration, piety doesn't matter.

In both games miss chance reduction is ceased after reaching level 20 and the number of attacks in the new class is independent of the levels in the previous class.