There is some (anecdotal) evidence of transmissions on their last leg
failing soon after a fluid change. Presumably debris from the friction
material and other junk is helping the old seals hold.
The seal softener that AC is talking about comes in a small can, so
shouldn't be a problem to simply pour it in. To make room for it, you can
suck a pint or so through the filler tube, using one of those little drill
pumps or some such. No need to change the fluid at that time. Years ago when
I tried that trick on a transmission with symptoms like  yours, it made
little difference, and the transmission failed completely a few months
later. Hopefully, you will have better luck.

To change the filter you need to drop the pan. About 1/3 of the fluid is in
the pan and only that quantity will need to be replaced. Unless the filter
is badly clogged with debris, changing it will probably make no difference.
But perhaps it's not such a bad idea to do that, because you'll be able to
see how big is the pile of clutch material shavings in the pan, and that may
be a clue to your tranny's remaining life.

Your transmission is on borrowed time. Enjoy while you can!

Miine was slipping on my '91 because the fluid level was low because a seal
was leaking.  I replaced the seal, filter, fluid, gasket, and added some of
that Lucas Trans Fix with the new fluid.  So far it has not slipped again.
This is my second transmission and it was supposedly a rebuilt but I have to
way of knowing they didn't just stick an old trans from a junk yard in
there.  Like Ashton said you might have a leaking vacuum modulator.  If it's
burning trans fluid your level could be low.  You can check the vacuum lines
on the cluster on the intake manifold and look for signs of trans fluid
inside the lines.  If you are lucky maybe it's just the modulator.  Or a
leak.