The chuck 'teeth' are worn down from using it as a hole saw
and I normally remove the chuck to put a drill guide on it.

These days I can't get the chuck out to repair or replace
the head.

The usual tapping of the chuck key on the opposite side
with a mallet doesn't work anymore. It's stuck.

My directions for the drill guide say that the owners manual
for a reversible drill has the answer to this, but I have
none.

Any clues on this besides the chuck key tapping trick?

Perhaps I had the forward/reverse switch set incorrectly.

In any case I can't fix it without maybe going to Sears.