In Wiz 6 it's determined by starting class, race and stats, and AFAIK in 7 and 8 it will be adjusted based on current class and stats. So starting with a Priest and changed to a Valkyrie is a good tactic in Wiz 6.

Incidentally, in Wis 7 it's generally better to stay with base classes longer to get more skill points, which are more precious than in Wiz 6.

Wizardry 8 is mechanically very different from its predecessors, to the point where a 1 to 1 character transfer would not be possible. For example, Wizardry 8's minimum starting stats are higher than the maximum possible stats in Wizardry 6-7; 20 is maximum in 6-7 IIRC, while 25 is considered very low in 8. There's also the fact that there's no Karma stat in 8, but there's a stat or two that *wasn't* in 7. Then factor in the change to how multi-classing works, and other changes (like Bards losing access to spells) that were made between the games, and any character transfer will have to rebuild your character from scratch.

So, I wouldn't expect a hidden value such as SP regen rates to carry over. (In any case, SP regen in Wizardry 8 I believe doesn't depend on class, and is much higher than in 6 or 7 (to the point where resting until your SP has recovered is actually a reasonable course of action in 8, even late game).

In any case, you don't keep your levels from game to game anyway, so you should really optimize your characters for the specific game your are playing (though, before transferring to Wizardry 8, change to the class you want to use in 8; Bishops may not be that good in 6 and 7, but it might be worth ending 7 as one just to use one in 8 where they're probably the most powerful class).

In 6, it's determined at creation and never changes. Each school has its own regen value. Pure casters get 2-3 across the board, except for the Bishop which gets slightly less than the pure casters. (E.g. mages get 3 in fire, water, and divine and 2 in the others; priests get 3 in earth, air, and divine and 2 in the others.) Nonmagic classes and hybrids get 1 across the board. 16 vitality at creation gets you +1 to all schools. Faeries also get +1 to all schools.

In 7, it's the same basic system, separate school regen values fixed at creation and never changing, but the numbers are different. Nonmagic and caster class values are mostly unchanged, but hybrids get 1-2 depending on school instead of just 1. In 7, both piety and vitality affect the regen at creation, with piety having double the effect of vitality. For example if a Mage wanted a +1 bonus he could have 16 piety and 10 vitality, or 15 piety and 12 vitality, and so on. It's theoretically possible in 7 to get a +2 from stats, but you either need 18 piety and 15 vitality, or 17+ in each. Bishops now have full caster regen instead of a penalty. Faeries still get +1. (Max regen for a faerie bishop would be 6 in fire/water/divine and 5 in earth/air/mental, but would require rolling a 25 at chargen for the stats, which is less than 1% odds normally.)

When importing 6->7, your mana regen gets recalculated, but your stats also get a slight nerf, but in all likelihood you'll get better regen in 7 than in 6. The game doesn't remember your previous classes, only the max level you've reached before class switching, so if you switch a priest to valkyrie in 6, it'll import using valkyrie regen values.

Regen in 8 is based on your piety, and also the number of spells you've learned for a given school. However, there are a large number of mana regen granting items in 8, which generally have a much larger effect on your regen than anything else. Being a faerie is a more noticeable increase than having 100 piety is, and even +1 mana regen from an item is significantly stronger than the faerie bonus. When you import 7->8, however, your stats are AT MOST 15 points higher than usual (out of a 1-100 scale, across 7 stats) so the benefit from importing is negligible. Importing is still useful because your combat and utility skills will be much higher than a starting party and you can bring some very useful midgame items along, but importing provides only marginal benefits for spellcasters.


TL;DR: It's very very helpful to start hybrid classes in 6 as their pure caster variant and then swap later, but it doesn't matter nearly as much in the other games, especially when you're importing. Class changing is extremely strong in both 6 and 7, but you're usually crippling yourself if you do it in 8, so make sure to finish all your switching before finishing 7.

"[i]...
When you import 7->8, however, your stats are AT MOST 15 points higher than usual (out of a 1-100 scale, across 7 stats) so the benefit from importing is negligible
....[/i]
That is not entirely correct, I try to explain:
The sum of all attributes will be up to 10 higher for an imported character than for a new created one.
The "attribute conversion" process on import focuses partially on the characters class related main attributes,
and likes to add some on top of the racial base requirements,
the remaining part (estimated < 50%) is evenly distributed with unclear system, probably some randomness involved.

For a character with straight forward class of few attribute requirements and similar race
this means that a fresh created PC will max their main attribute to 100 and the related powerskill earlier,
while e.g. a ninja with tons of class requirements usually benefits in a way that reflextion and snakespeed are earlier unlocked.

The main advantage of imported parties is the wide range of skills at up to 20.
Skills import at a rate of 1/5, so best max whatever possible to 100 in Wiz7.
Since characters import to wiz8 at level 1 with free xp points,
new specialist casters often have a higher school skill at 13k XP than a imported specialist caster,
while an imported bishop with all schools and realms at 20 has a really huge advantage vs a new created one.

I agree that class change is mostly suboptimal in Wiz8,
but an imported [e.g. ranger] who intantly swithes to gadgeteer is better than a new one.