as i understand it, you having bought the product are entitled to two “single unit” for lack of a better term licenses…in otherwords you can load z2 on two computers but only if you use one computer at a time…however it behooves you to save the info that zbrush asks you too when you first load it on the first machine…ie the directory all the password and important info is stored in the first place.
if you load z2 on one computer only then you can surely reload it again without problems in theory should you have a crash.
if you change computers or systems or whatever then…contact pixologic for the how to’s …they are pretty fantastic folks about taking care of their customers so I reckon you’ll have no problems getting things up and running on your new system quickly.