I see no mention of Micollab so consider yourself lucky it's "that easy".
Depending on how users are dispatched on the MICD it might have been a good idea to convert some of the mivbs "blades" to have all those users transferred for free, then you just have to consolidate. But I don't know if it's possible to convert from MICD, it probably is, but documented, I don't know. I think I've seen something about that when looking for information on MICD (and realizing it's a dead product).
A few years ago I did 4 MXE Server to 4 ISS, and there was no documentation, we had to improvise, it was fun. However we kept the number of machines identical so no need to work on users.
We still have the ISS and are now removing about 50 MXe from our cluster, consolidating all users on the ISS, and we do it manually : export the users, export the keys, and import on the ISS. Tell the users beforehand that they will lose stuff and ask us to program it back if needed...