"Steve B" wrote

OR, ... DO IT YOURSELF! We're not talking plumbing here.
We're talking about changing parts. There's no plumbing
involved in changing a flange gasket. And not much
involved in changing a flange.

Suppose a year from now I needed to change the flange, how
would I remove the old one easily? Remember, it is made of
plastic and glued to the plastic upright pipe that comes up
from the sewer line, using the same kind of glue that lawn
sprinkler systems use to connect joints and pipes.

I hope I never have to do that, but it sounds like I'd need
some tool to fit inside the upright pipe and then cut the
pipe. Then, since the pipe would be ending a few inches
lower than before, I'd need a way to extend the pipe upwards
so the new flange could be installed at the same level as
before. (The upright plastic pipe comes up through a hole
in the concrete slab, and there is less than an inch of
clearance on each side of the pipe and the slab.)

Thanks for whatever ideas on doing this that you can share.