I thought some of the earlier Agnus chips could only do one mode, but maybe those didn't come in 2000s.
it did come on come A2000/500s.
maybe with 4.x revisions (although I do have one board hanging on the wall and it has a jumppad as well).
So in general the switching would not work if Agnus did not support it (Agnus 8370 NTSC and 8371 PAL found on some rev 4.x and rev5 and earlier A500s. 8370 was made in separate versions).
the Agnus is the exact chip in newer revisions (6 and up) : 8372A 318069-02 for pal and ntsc.
@Dannyp
Curious, why did you ask this question?