Multi-classed characters do not suffer experience penalties, but note
that acquiring the first level in a new class may be a lot harder than
D&D suggests.  That is, learning the base skills and feats may take
substantial time, feats, and/or skill points, rather than just a few
thousand experience points.

I'm allowing fraction-accumulation of saving throws and base attack
bonus, which are relevant and an advantage only for multi-classed
characters: Keep fractions, so +1/2 Fortitude save from each of two
classes results in a net +1 Fortitude save.  Also, the initial +2 for
a particular category of save comes only once, even if you have two
classes that give +2 at 1st level on the same save, so that a 1st
Fighter / 1st Cleric would have +3 to Fortitude save (not +4).

----
One of the weaker aspects of 3rd Edition for modeling Fivtoria PCs is
the difficulty a 10th-level Wizard / 1st-level Figher would go through
to pick up a second level of Fighter.  Instead of the 2k xp he would
need if only a 1st Fighter, he now needs 11k xp because of the weight
of all those years of spell casting.  But on top of that, experience
points come not from defeating orcs or ogres, but from defeating
dragons!  He gets no experience at all from defeating such
insignificant little creatures as a 1st-level Fighter could hope to
defeat[*].  Admittedly, the 10 levels of Wizard make him effectively a
6th level Fighter, since Wizard contributes about half as much to
basic fighting ability numbers as does Fighter.  But some compromise
is still needed for existing Fivtorian high-level NPCs to make sense,
rather than either have stupid configurations or be hand-waved.

[*]: As an aside, a character should get experience scaled to the
level of ability he used in the adventure, rather than scaled to his
actual character level.  So if a wizardly character cannot or does not
do anything but melee for a run, he'd count as about half his level
for scaling purposes.  This means high-level characters *can* get
useful experience for little things, though they might have to
restrict their actions to do so (quite a bit like "dual-class"
characters in 1st Edition).

A character who advances 5 levels in arcane magic (wizard) or in
fighting would need to have advanced 5 levels to do so.  But if the
two come together, the total levels needed are fewer than 10.  Why?
Because advancing at 2/3 rate in spells (or anything else) is about
half as good as advancing at the normal rate.  Therefor, the standard
"mixed" classes like Bard and Paladin, and even the Wizard, look like
they're getting more than one level worth of advancement per level.
The Wizard gets a full half-level in fighting ability for free with
his magic!  So, the main benefit of advancing a level in a class is
not just improving with that class' abilities, but improving as
rapidly as possible with them.

We could easily piece together any number of classes that had
reasonable advancement in any two of: combat ability, arcane magic,
divine magic, or skills.  By applying this theory, we can deal with
the high-level character who wants to pick up a new skill.

My proposal: That we allow levels to be retroactively swapped from one
class to another, as long as no abilities drop.  That is, the
hypothetical 10th Wizard / 1st Fighter, upon advancing another level,
could configure his 12 levels to be 6 of Wizard, and 6 of a wizardly
fighting class.  If this wizardly fighting class advances in magic at
2/3 the rate of a Wizard, then his ability with magic has stayed
constant.  But he started with 1 level of Fighter, plus effectively 5
more from Wizard (for a total effective 6th Fighter), and went to 6
levels of mostly-Fighter, plus effectively 3 more levels from Wizard
(for a total effective 8th Fighter).  So he increased about 2 levels
in fighting ability with one level of advancement.  Since a 6th-level
Fighter would have needed half as much experience to advance a level,
that fits pretty well.

So, all that is needed is the math for figuring out how much you can
stuff into a mixed class.  Conveniently, I've already done that for
the experimental 3rd Edition run I did for a while.  That, combined
with a lack of experience penalties for multiple classes, means that
this conversion problem should be solved.

BTW, this *could* be made to also impact the concept of a prestige
class.  If it is possible to infinitely reconfigure your levels as
long as you don't decrease any ability, then old levels might be
convertable to prestige class levels after the fact, as long as all of
the requirements of the prestige class are still met.