just for a peace of mind.
just in case you H1B transfer is not approved, then you do not go to the new
employer and you can keep working at the current employer.
If you move to the new employer and later on you find out your case is
denied (I know the chance is really small), you previous employer may have
notified the USCIS that you no longer work for them anymore. in that case,
you cannot go back to your old employer to work anymore.