Brief update.
Spent last week correcting minor issues in Waste's Edge itself, then took Saturday off, as I started to feel a bit worn out by all the testing that every small change brings with it.
Today I came back full of energy and replaced SDL with SDL2 in one fell swoop. I now have it in the state where everything compiles again, but quite a bit of fixing (and more testing) is needed. But with most of the work out of the way, I can now gradually iron out the issues over the coming week, so I think everything is right on track.
Unlike with Python, I dropped SDL 1.2 support completely. There are too many subtle differences that will be easier to address if I don't have to plaster the code with #ifdefs. Besides, the 0.3.6 prerelease tarball would still be available if somebody absolutely must have SDL 1.2, and I might tag the repository before committing my changes. I will also ensure that Waste's Edge itself remains unaffected by the engine changes.
All in all, the transition went smoother than expected, partly thanks to the work already done in v0.4.