I would clone the hdd to the ssd while still on 8.1. Depending on how much data you have on your hdd and how big of an ssd you get you may need go re-partition the hdd in order to separate your OS and programs from your data. You will want the OS and programs partition to be smaller than the ssd you wish to use.
You shouldn't lose anything in the process but, as always, make a backup BEFORE you start messing with your partitions, just in case.