Do you have any control of the server? ie: your own dedicated box that you can throw whatever os/zcs version you want on it?
If transferring your entire setup is impossible-as will probably be the case if it's not a dedicated box for your use...
Till someone makes the zcs>zcs tool
Bug 19630 - Migration Tool zimbra to zimbra
(but that probably wont happen soon because the network edition has zmbackup>zmrestore)
your left with, scripting some account provisioning + imapsync + all the calendar data exports/imports & what not...
(I would hope the hosting provider -whomever they may be- would work with you for the transition...)