Well your aperture library is actually one single, massive file. Which contains basically all your photos and data. If you trial'ed 3 your safest bet is to change your Library save location to somewhere different from your original and then just go File - Import - Library/Projects.
Select your old library and it'll say "This Aperture Library is an older version and needs to be converted"
It'll take sometime converting it, and there you have it, old Library still intact and unaffected and new Library running in A3. When the trial runs out any images you import into 3 though will import into the new Library and not the old one, so if you decide not to continue export the Masters you have imported since using A3 so you can import them back into A2.
I was a long time A2 user, switched to A3 and initially didn't like it, but that was just me being used to the way A2 worked.
A3 has better processing powers, and overall is nicer and better. Takes a while to get used to it, make sure you give it a full go before going 'meh'