Quoted from BJM-Maxx:
I have heard of that method, how hard are these things stuck on, am I expecting to tap hammer, or use some solid force?
Ok, so medium soft taps are best, but you may have to use force if you get no budging. Also, if there is a lot of rust, I have had great luck spraying a SMALL amount of WD-40 on the front of the glass so it dribbles into the front of the channel. Let it soak a bit and then try your tapping again. By all means be careful, you do not want WD-40 on the artwork, but it really does help dissolve the rust which reduces the pressure on the channel. Wipe it off after you get the channel off.
Originally the channels are just pressed on and there is a bit of friction tape or two sided tape holding it on. It's the rust and years of crud that make it really tight usually.
BTW: I haven't broken a backglass yet doing this and I've removed like 20 some channels and put new ones on over the years. I actually lay my hammer down on the front of the backglass (on a washcloth) and just slide it into the channel/wood block. I don't lift and 'hammer' on the glass. If it's a really nice glass then I'll put some blue painter tape on the front, but I use a smallish ballpeen hammer so it's got a round head anyway.