As promised, here's a brief photo tutorial on the T5p surgery:
Step 1: Grab the edge of the earpad and gently pull up. It should come off without resistance.
Step 2: Using a knife (any knife with a thin blade will do), insert the back side of the blade into the slit and pry sideways as marked.
That will detach the fastening ring.
Step 3: Using the knife blade, apply light pressure to the raised side of the driver assembly and pull away from the cup. Sounds scary, but it leaves hardly any marks and is not visible anyway.
The cable inside should be long enough to allow for some movement, nevertheless apply some care unless you are ready for some emergency soldering...
Step 4: Make sure the foam rings fit the inside of the cup.
Step 5: Insert the foam ring. No gluing is necessary.
Step 6: The earcup contains a groove on the side of cable fitting. When assembling back, align the tongue on the driver mount with that grove and gently press the mount in.
Step 7: Fit the earpad on the fastening ring. This can be a bit fiddly. I use a corner of a credit card to guide the edge in.
Step 8: Find the small tongue on the edge of the ring and align it with the groove on the earcup (yes, the same groove as before). Squish the two together. You should hear a few clicks. Make sure that the fit is nice and even around the edge.
Well - that's it. Hope this may encourage some of you to experimenting and if you do - please share your findings!