Really it's a no-brainer, well a lot easier than converting old-new.
Divine Magic converts as is, Progressive becomes Stackable, otherwise no real changes. Forget Dedicated POW and Lore(Specific Theology) as they didn't exist.
Sorcery pretty much converts as is.
Runemagic becomes Battle/Spirit Magic. Use RQ3 chances to cast, ignore any and all Runic Affiliations, allow unlimited variable spells (Bladesharp 20 etc).
Enchanting is different - I'd stick with RQM enchantments as they seem better than the RQ3 rules.
Use RQ3 Spirit Combat rather than the abomination that is RQM Spirit Combat.
Characteristics stay the same (APP for CHA in RQ3).
Skills need a bit of work. Generic skills such as Athletics need breaking up into Climb, Jump. Swim, use Run as a new skill as well, if you want. If you are converting RQM to RQ2 then there are even more skills to use.
Hit Point, Fatigue, ENC and SRs all need recalculating, but that's a one-off thing using tables.
Weapon stats need changing, with damage, AP and ENC differing. Weapon skills need tweaking as RQ3 has Attack and Parry being different skills, but that's a GM choice, or a GM/Player discussion point.
Non-humans can be ported across, or use RQ3 equivalents.
I'd use the excellent rules in RQM for altering armour and equipment, they are better then the ones in RQ3.
That's all I can think of at the moment, I am sure I've missed a lot off.