I think the Downloaded Transactions will still be there after the update. You can always try it and see. When it updates to a new version the old copy of your company file is untouched - it just gets renamed and left in the same directory. So you can go back to 2015 and clear them up if they aren't preserved. But they should be.
That said I wouldn't leave them in there very long myself. A change to the bank's account number (or credit card number for credit cards) will invalidate all of them. That could happen at any time. Best to clean them up. Besides the last year is over and you want to reconcile all your accounts with your bank statements before you pay any taxes. Otherwise you might submit erroneous info and then its a real hassle to correct with the IRS.
The 2019 version can currently only be registered by calling. The online option isn't working due to a change in the registration server. That should be cleared up pretty soon. You may want to wait until then (although calling does work it takes 30mins or more sometimes).
The 2015 version had issues initially when Apple got rid of Garbage Collection. I think that was Sierra - don't recall exactly. We fixed it as soon as we could. But there was an initial period where it wouldn't launch on Sierra.
I encourage you to update to the latest version of 2015. Its been out for years now (it was released in 2014). The newer releases will be more reliable than the older ones.