when the rule was originally introduced level 20 was the max (the server has been around since before the expansions were released), now however save/attack bonus is uniform for every character class after level 20, so a character that leveled up to 18 fighter before starting on cleric path could be seen as trying to max out his attack bonus since it's going to go up by 1 per level regardless of the class he picks after 20
The example you give (fighter to cleric) probably wouldn't be a problem, so far as I know. But as a CA, I don't want to have to deal with every single possible multi-class situation, and decide whether something would be a problem or not. Having a rule like that to apply across the board - whether or not it in the particular case the rule is serving its purpose - just makes life easier. I don't want to have to have a catalogue in my head of all possible power-builds that we don't allow.
It also helps make sure characters actually have RP investment in their classes, rather than just taking a level or two of certain classes for mechanical benefit.
Mechanically enforcing five levels in each taken class before 20th ensures a significant investment in each class, and the mechanical investment will be enough that it reflects in the character's RP before the character reaches epic status.
In fact, in certain situations, this hinders roleplaying. Say that a paladin does manage to do something to become a Champion of Toran. From the rules as stated, if that paladin happened to be high sub-"epic", they could not actually become the Champion that they are, even if they intend to be a champion for the rest of the levels that they gain. They would have to remain a paladin until level 20.
Well it sounds like there is some flexibility allowed, which is what I was looking for.But as a general principle, I think that some of these hard rules to try to stop "twinkie" or "munchkin" builds are worse than the disease. First, because they cut off legitimate choices of growth for characters. And second, because they don't actually do much to restrict munchkinism.Power gaming in a low XP world has little to do with "builds" at all. Instead, all you really need is a reasonably efficient wizard or fighter and a single minded devotion to the commitment of genocide against the race of giants. And none of these class changing restrictions - not even the 5 levels in a class one - hinder that.
But just so that this isn't a surprise later, if the "flexibility" you're seeking is considered critical by you and your character's future, it will take a very good (in the eyes of the CA and GM Teams), non-mechanical justification for the CAs to approve it.