I agree with Techknight, keep the CRT and recap the analog board. Screen Burn is only an issue if if interferes with seeing what is on the screen, and that is rare to have. Most of the time it just has a burned image of the menu bar and maybe, just maybe a couple of icons, nothing to worry about.

Swapping the tube from the classic to the SE/30 is too much of a hassle as you have to exchange the yoke and a few other things  and that requires constant adjustment if its put in off by just a bit. At least with the SE, you can swap out the whole thing in one shot.

What's wrong with the SE that it became a Donor Machine?