The CICS systems respond far better to upgrades than MICS systems do- they store thier programming on the backplane rather than on the software card. That being said, I have encountered situations when I've lost partial, and once, all previous programming. If you have a fastrad, and Nortel's backup/restore tool, I would suggest using it before you upgrade. If not, sitting down with pen and paper to copy down your system programming is time well spent.
Anyway, if you have to upgrade, I would reccommend going directly to ver 7.0 .
Good Luck.