@LordAga
Cos the A500+ allways has an Agnus capable of addressing 2MB chip, so all 1MB expansion were made to appear as extra chip.
Non+ A500 might contain an orginal "Fat"Agnus only capable of addressing 512KB chip, so the trapdoor was designed to host 512KB of SlowMEM.
Later revisions of the A500 contained either an 1MB Agnus or even an 2MB Agnus (these were really just downgraded A500+ mobos) at which point it made sense to use the extra 512k in the trapdoor as chipMEM.
If you A500 came with an 1MB Agnus standard it's as simple as changing one solder-jumper (JP2 left of the 68k AFAIR) and switching the 512k-expansion of (so it wouldn't be regognized twice).