That's what I did to separate the metal cups from the headphones originally. I poured boiling water into a metal cooking tray, then set the metal cups into the water (about 1cm deep so the grill was submerged but it didn't reach the drivers), waited a couple of minutes, and it melted the glue keeping the driver cups inside the metal cup. Clearly, this isn't the safest method though. It worked fine for me as I didn't have a hair dryer, but I would suggest using the hair dryer if you have one available.
I tried the same thing for the button glue, I removed the grills and buttons, then submerged the grills in boiling water, but that didn't do the trick and I had to resort to the fireplace to get the glue off completely.
You might wanna try rubbing alcohol, vinegar, or a commercial glue remover. Your best bet though, in my opinion, is to just go out and buy a sheet of plastic grill, cut it to size, and replace it entirely. It'll be much easier and it's what I should've done myself for my 325s.