The configuration and features of ArubaOS 8 in standalone mode do not exactly match what is in version 6. Some things work the same and some do not. I never had to make that comparison, because I don't have much experience with standalone mode or encounter customers who run it. I do know that it can be very limiting vs. running a network with an MM.
I do not know anyone who has successfuly migrated from standalone mode to an MM; they just waited until they got an MM so they would not have to start from scratch.
You can leave the controllers on AOS version 8, because it would get you familiar with aspects of AOS8. When you get an MM and add the controllers, you typically have to "write erase all" and run through the startup wizard to add them to the MM; which means the configuration on the standalone will be replaced by whatever is on the MM...