you just need to pay the difference. if you want to upgrade, just let me know which package you want and I'll send you a money invoice though PayPal

when you say "premium" version, which version you mean?

I would like to upgrade to this one:

http://www.joomlamusicsolutions.com/component/ambrasubs/subscriptions/new/10.html?lang=en

ok, I'll proceed to send you the money invoice. once you process it we'll upgrade your account!

When I upgrade will it mess up all the customization I have done to all the template files?

yes, if you perform a fresh install...
but you can avoid this simply making a copy of your /views folder, and then pasting it to replace the "fresh install" copy.

Sorry for the dumb question but what is the best way to upgrade from standard to pro?

When I try to upload the zip file it fails and says there is already a com installed there...

uninstall and then reinstall. that's the way.

make a copy of all the layout files you want to keep, so you can replace later!

Will I loose all the artists?

nope. you won't loose artists or albums or songs.