*Remove the cover and change the ballast or just change the entire
fixture.
Rapid start and preheat bulbs are not interchangeable because they have
different pins on the ends and require different sockets for each.

That is NOT TRUE. There are even F40T12 bulbs rated to be compatible
with both. Nowadays, F40T12 bulbs are generally compatible with rapid
start ballasts, since nearly all F40T12 ballasts have been rapid start for
a decade or two.

I have seen maybe 3 different ends of 4-foot fluorescents, and all that
were different from the usual 2-pin ones were fairly specioalized instant
start lamps requiring according ballasts.



*You are correct. After I posted that I realized that I was thinking about
instant start ballasts which are wired through the lamps and the pins are
different.