The first hit I get on google for Tri-X 400 Pan tells me:

KODAK TRI-X Pan Film has been replaced by KODAK PROFESSIONAL TRI-X 400 Film / 400TX.

The second hit is about Pro Tri-X 400, and that says:

Compared to KODAK TRI-X Pan and KODAK TRI-X Pan Professional Film, the newer TRI-X 400 and 320 Films may have a slightly different retouching “feel”.

At a quick glance, I notice that the timings in development charts is slightly different. So, it is a slightly different film.

The first hit I get on google for Tri-X 400 Pan tells me:

KODAK TRI-X Pan Film has been replaced by KODAK PROFESSIONAL TRI-X 400 Film / 400TX.

The second hit is about Pro Tri-X 400, and that says:

Compared to KODAK TRI-X Pan and KODAK TRI-X Pan Professional Film, the newer TRI-X 400 and 320 Films may have a slightly different retouching “feel”.

At a quick glance, I notice that the timings in development charts are slightly different. So, it is a slightly different film.