Changing OCC mid game
It happens in real life so why not in rifts? Heres how I handle it:
If to a T-Man just follow the rules as given. But say a Ley-Line Walker wants to become a Techno Wiazrd no problem just freeze all skills he has right then (These will not start advancing until the Charater reachs the same level as he was before the change ie, A 7th level Walker becomes a TW his skills he learned as a Walker don't advance until he is an 8th leavel TW). Gain the OCC skills of the new class, and also half the number of of the "OCC related skills." If there is something contractictory between the to class, say the original was a magic user but the new occ was not a user, say a roge scholar, the character cannot cast spells until the has surpassed the level he was as a magic user. Limits the Charater cannot change classes until it is at least 2nd level. Also to gain the skills at least 6 month of training must take place, 1 year for a character with no magic to learn magic, but if the Character has an IQ score of 20 or more reduce time by half. The Change cannot be done with out training but this training maybe aquired while adventuring ie the groups techno-wizard teachs the rogue scientist with the high IQ in all spare moments basicly. When the Character has surpassed his previous level he can go back and pick up the study again without training or skill addition. If a character ever begins to train in a class that he already had he can no longer change classes. note use ppe gain rate of current class.
(note this is a modified system of the AD&D rules for changing class)

Symbiots for the Supernatural
Symbiots making normal humans to powerful for the dragon character to keep up? Simple just charge 50% extra and call it a experiment and warn them of side effect that might occur. If any side effects do occur I leave that up to the GM's descretion.

New Allignment
Have you been looking for something more like the Dungens and Dragons Chacotic Good allignment? Here's what I use: take the information for Aberant Evil and call it Aberant Good. The character tries to do good and help people like a scrupulous person but reallizes that in this world Torture, Kidnapping, Lies and Killing (all though distactful and disgusting)can be and are tools that must be used from time to time in this world.

Multiple RCC
A rule I only use ocationaly this would be like a Rahmen burster or a Chaing-ku child of the stars. Only things like this because one is a species and and one is caused by hard work or purely enviromental sercumstances. This cn only be done during Charater creation. First and formost ALL WAYS have the GM's permition before doing something like this. The Character now uses the Dragon Experince table and can follow the rule for changing OCC if the GM alows and if one of the RCC's could consiveable switch classes.