The foam dampers mod is completely safe and reversible. When you take off the ear pads, you will see the foam rings are around the driver, and the driver is protected by a metal grill. It is not possible to screw it up. The foam rings are just fit in and are removable. The great thing about the TH900 is that each and every small part can be ordered via the importers (there is an exploded view of the headphones with all parts code on it). And by the way, IMO the TH900 is very well designed in all details, they absolutely knew what they were doing.
The ear pads mod is irreversible, but the replacement pads are relatively cheap. I ordered a replacement pair together with the white plastic attachment plates, on order to easily change pads based on my mood, and whether I am listening to music or movies.