You may want a professional to install a thru-hull transducer,
since, if something goes wrong, your boat may sink.
I guess the main thing is getting the hole drilled straight, and at the
proper angle.
They will probably use 5200 marine sealer on it, since it's
for below the water line sealing.
If it already has that sealer on it, it will be very difficult to remove,
but not impossible. You could use a razor blade to cut the adhesive,
and a long nosed vise grip to grip and pull it off in pieces.
I've used that stuff on my boat for 10 years, and it's great.
I never look forward to removing something that was secured
with it, but so far, I have managed to remove everything
I wanted to.
The stuff gets dirty with age, but will last forever, especially
if you paint over it.
It's very messy to work with.
It would be nice if you could get some kind of adapter to use
the lowrance transducer with your Humminbird.
Maybe someone else knows the best way for you to proceed.