I have done it twice. The first, I took a really sharp knife with a flexible blade and just went over the plastic until I made it through. The second time, I used a planer until I was pretty sure I was almost to the glass, then used a sharp razor. A 2.5 has very thin glass so you have to be extra careful. I was doing a 20 long which is probably much easier to not break.
The hardest part by far is cleaning off the old silicone. I used rubbing alcohol and about 10 razor blades, only doing the front really well, the sides OK, and not doing the back. I only took the top rim off. I would think you would need at least 10 razor blades and a few hours to get it done right on a 2.5. It's really hard to get right, takes time, patience and a bit of skill to not scratch the glass.
I also finished the front and side glass using a very fine file and then some 400 grit sand paper, moving up to 800, then 1200. I just happened to have a lot from another project. That took a few hours as well.
That said, in retrospect, I would probably not de-rim a tank again. It's a lot of work to do. It would have not been much cost prohibitive to buy a rimless (Mr. Aqua) tank. The time it took me to do it was not worth it, outside of one already having a stand already. Even if you do it right, the silicone work still doesn't compare to a cheap rimless tank.