"Larry Weil" wrote

this could be used with an existing toilet, and he said it could if the
hole pattern matched for the bolts that hold the tank to the bowl. He
suggested that I measure the spacing of the bolts on the existing toilet
and then come back.

This is partly correct.

said that I should not purchase a new tank even if it had the same bolt
spacing. He said the salesman in the first store was wrong, that even
if the bolts match the tanks should only be used on the bowls for which
they are designed, since they all sit on the bowl differently, and may
not seal properly and thus may leak at the gasket between the tank and
the bowl.

This may also be correct.

The answer depends on if the problem is partly the design of the bowl and
how close the design of the new unit's bowl is to the existing one.

Now, if you can get the base portion separately (and be sure they dont run
out of that model before you get back!) it's possible to try just the tank
back on the existing pedistal but be prepared to have to go back and get the
base then redo it if it doesnt work.

We were able to do what you propose with one of our 2 toilets. The other
one, we had to replace the whole unit because the bowl was 'wrong' and
flushing was inadequate with the origional bowl.