A: No. If you try and boot an AP2.5 protected backup patched for LT+ 2.0 on LT+ 3.0 or vice versa, it won't boot and you'll flag your console
From now on, it is highly recommended you do the following: -
1. Update to the latest dashboard and flash to LT+ 3.0.
2. Patch all AP2.5 backups using the topology data.
3. Re-burn your newly patched backups using Burner Max Memorex and Vebatim media.
You don't need to reburn older games that don't have the AP25 security. AP25 is an additional security sector on the disc. The first game that had this physically different disc was Fable III, followed by NFS Hot Pursuit and Assassin's Creed: Brotherhood.
For any games before that, the retail disc did not have an AP25 sector. The only games that you would need to reburn would be one of the new AP25 games (the three I mentioned plus whatever other new games) that you burnt before abgx360 was updated with the ability to do AP25 patching, because those won't pass AP25 checks and will result in your xbox being flagged.
All older games work perfectly since those did not have AP25 to begin with (and it can't be added in unless the game was re-released, with newly pressed discs).