They're all Red Skeletons with the Archmage background option.
I sent one of my groups against a Red Skeleton. It was a blast!
But to the main topic, I don't allow class changes for the same reasons Corey Magel listed in his reply. The DP costs are set by professions, but the beauty of RM as compared to another popular RPG, any PC can learn any skill... at a cost. Fighter wants to learn a spell list? 20 DP for a 5% chance. He can do it, but he won't be too good at it. If I use a Fighter for about 5 levels, I'm looking at great fighting skills and bonuses. If I switch to a spell caster, I have extremely low cost magical abilities, with already high combat skills and who will undoubtedly become a killing machine in a few levels. Extremely unbalanced and OP. Not many of our players buy skill ranks past the first ten ranks anyway, at level 4, the primary combat skills are probably maxed out.
Changing from Cleric to Evil Cleric I wouldn't allow either even though it seems like a logical shift. However, one class can learn spell lists from another class, again with a cost. If the player wanted to play an Evil Cleric, he should have just rolled up an Evil Cleric from the get go. "Evil" is a relative term anyway. Just because a PC has the title "Evil Dude" in his name it doesn't mean he goes around killing every singly human being he meets or wears a sign that says "I'm evil, stay away from me." Quite the contrary, the evil person will want to fit in and to draw as little attention to himself as possible as he practices his nefarious ways.
In our gaming world, necromancy is frowned upon and considered "evil" but when there's a Lich attacking a town, the necromancer is sought out for help. They are like the lawyers of today... a necessary evil
The evil cleric can still be played as a "good" character, he just has access to different spells. It's how those spells are used that defines evil. The evil spell lists could be used against an attacker as the Evil Cleric tries to protect a group of missionaries. A Healer can be just as evil as an Evil Cleric. Keep healing the bad guy so he pounds the crap out of the Healer's enemy. A Healer would have Poison Lore and access to poisons and could easily disguise the manner in which a person died.
What determines "evil" is intent and implementation. How many times has the Priest leader of a cult turned out to be the bad guy? To his followers, he wasn't evil, everyone else was.