Hi netcoach1
You will be sent a migration letter inviting you to apply for UC, you will then have 3 months +1 day in which to apply if you do not apply then your legacy benefits will stop (IR ESA and HB) not your PIP.
When you get the letter we would advise you to contact your local Welfare Rights organisation;
advicelocal.uk for some claimants, timing is important when applying for UC.
Gary