The file that you are looking for is "ArchiCAD 8.* Lasting.PRF".
I just tried renaming and substituting the current "8.1 Lasting.PRF" with and older "7.0 Lasting.PRF"... Doesn't work.
Unfortunately, I no longer have an old 8.0 PRF file to test this. But I presume that a renamed .PRF file created in AC 8.0 should work in AC 8.1.
David Larrew, AIA, GDLA, GSRC
Architectural Technology Specialist
a r c h i S O L U T I O N S
WIN7-10/ OSX 10.15.7
AC 5.1-25 USA