Basically - anything that works on V12.6.X will work on V12.7.0 (and 12.8.0, and 12.9.0), except for the custom pickup skill objects. However, V12.7.0 content generally won't work with V12.6.X - it's backwards-compatible, but not forwards-compatible. If you prefer, you can continue working on your LWT conversion in V12.6.5 / editor V1.14, then just update the levels to V12.7.0 later - at the latest, during V12.9.X's time, but I would suggest don't put it off to the last minute, especially since this is a very easy task. (For reference - across all my packs combined, I found four broken replays. Two were due to trigger area changes, and two were already-broken replays in unreleased levels that I hadn't picked up on before.)
I'm not at all advising against updating your content - to the contrary, I advise that you do update the content, but just hold off on releasing it until V12.7.0-stable is released (sometime next month, probably more towards the start, all going well). In terms of updating levels, this is very easy - just run them through "Cleanse Levels" (press F8 on title screen, then look in the "Cleanse" folder), run a mass replay test just to be safe, and you should be good to go.
The one thing you'd need to do manually, is restore any anti-splat pads you previously had in your levels and want to put back. Automated tools aren't smart enough to pick up on this, unfortunately. EDIT: And, if you're using custom pickup skill objects that haven't yet been re-created for V12.7.X, you'll need to manually handle the levels that contain them. Likewise, if you want to switch from defaults to custom ones on certain levels (eg. any levels using LPVI styles, as these have had custom pickup skill objects added now), this also needs to be done manually.