Peabody wrote:
....
Is it reasonably
practical to desolder the old part, and re-solder the new one,
without destroying the plastic?

No.

If not, I'm thinking I could perhaps leave the old part in place,
and parallel in the new one - if there's room to do that - and just
solder to the leads of the old part. Carefully.

Clip the leads to leave at least 1/4" to 3/8" of bare lead soldered
in. Get a heat sink, like a forceps or a big alligator clip, clip
the lead at the base, trim the leads of the new part, and just sweat-
solder the leads together.

Good Luck!
Rich