Does anyone know if there are problems with mixing ZCO versions with ZCS versions?

I'm mainly asking about:

ZCO 4.0.4 w/ ZCS 4.0.5
ZCO 4.0.5 w/ ZCS 4.0.4

and (not as important right now but will be once 4.5 GA arrives)
ZCO 4.0.5 w/ ZCS 4.5.0 ?

We are trying to figure out what order to upgrade server and ZCO users in, and also how fast do make the users upgrade their ZCO.

Also, is the recommended way to upgrade the ZCO merely a reinstall on top of the existing ZCO install? or does the existing one need to be uninstalled before the new one is installed?

Thanks