Hello tgx.
There shouldn't be any problem, but you might get some exceptions for Documents (as in Zimbra 7 such function is merged with the Briefcase).
Mind that using Zimbra's Import/Export features, both via WebClient and CLI, only exports data - no shares, no tags, no user's password and preferences. If you need to also export that kind of informations, you can take a look at
ZeXtras Migration Tool.
Have a nice day,
Cine