Only problem is that valve changes the path of the freon. So the
freon in the outdoor unit would have to be recovered (hopefully there are
hand valves that isolate it from the lines) the valve replaced, vacuum
pulled and then recharged. Also I think there are two such valves unless
they've redesigned them with only one more recent than the 25 years ago I
worked on them

Don't worry, John, there's no way I would try doing this one myself!
When it comes to DIY, I have a rule: if it can blow up and kill me,
don't mess with it! ;-)

Based on the symptoms I posted, does a faulty reversing valve seem
like a reasonable diagnosis? I can save myself $100+ if I call the
repair guys and just tell them that I need a quote on replacing this
specific part, instead of having them come out to look at it
themselves.

- Jason