There were two unused bytes on the direct page between Floating Point Accumulator 1 and the current execution line number.  MATHTEMP (added to speed up the math library) has been moved there so that all direct page variables are now at the same location as the original ROM.  As long as a program doesn't try to use the two MATHTEMP addresses, there shouldn't be any incompatibilities on the direct page.
That was going to hold the next line pointer, but that's going to be a new revision so this one might as well be as compatible as possible.