It should be reversible although how much work it will take will depend on the upgrade method. I had to desolder and replace all of the DRAM on a de-upgraded 128k I have to bring it back to original condition. If there were snap-on upgrades for the 128k (I know there were several for the 512k), you may be able to get away with simply removing an add-on board from the logic board.
The big question is - does it have it's original ROM chips and 400k floppy drive? If it still has the original 400k drive, you should see a little red light when looking down into the floppy slot while it is turned on. The original ROMs should be numbered 342-0220/342-0221. If either have been upgraded, replacing those with original parts are going to be the biggest cost of reversing the upgrade.
The big question is - does it have it's original ROM chips and 400k floppy drive? If it still has the original 400k drive, you should see a little red light when looking down into the floppy slot while it is turned on. The original ROMs should be numbered 342-0220/342-0221. If either have been upgraded, replacing those with original parts are going to be the biggest cost of reversing the upgrade.