A friend has a pending sale (Reverb) on her 1951 Gibson J-185. It will need the bridge replaced with a repro bridge. It seems the pending buyer's luthier has concerns that the current bridge may glued on with some sort of Epoxy glue. Is this a serious problem in terms of getting an old bridge off?
Short answer: No.
Long answer: see below...
The benefits of animal protein glues (hide, fish, etc) are well known: they are anti-creeping and will readily release their bond when heat is applied. These benefits have proven to be helpful, especially over the last century in the development of the steel string acoustic guitar. The instruments disassemble easily, making prototyping, let alone repairs, so much easier. For example, when removing a hardwood bridge off a softwood Spruce or Cedar soundboard, if hide glue was used, there is little chance of taking sections of the soundboard with it.
The downside to animal protein glues is found in their extremely short open, or working, time. This is the period of time (counted in seconds), that the crafstman has to position the part(s) before the adhesive bond is formed. Parts assembly must be coordinated with the open time of the glue in use, which can be rather restrictive. This establishes the proper use of animal protein glues in lutherie as more of an art form than a simple skill to master (think: French Polish shellac vs wipe-on varnish). There is also the issue of the quality of the glue (like Shellac: short shelf life) along with the economics and practicality of keeping sufficient quantities on hand, ready to go. The heat at which these glues will soften and release is relatively low, as many have come to learn the hard way, thanks largely to the automotive industry. Animal protein glues are water soluable, meaning things built with them fall apart in moist environments.
By contrast, for the luthier (or woodworker or craftsman) more modern (wood) glues are very pragmatic. They have relatively long shelf lives, and can sit in a shop in a bottle, an arm’s reach away for immediate application. Their adhesive strength is significant, and the bond they form can be relied on. They can (with some care) even be coaxed to release their bond.
Epoxies are the exception. While I have actually taken an instrument apart that I put together using epoxy (just to prove it could be done), it is not pretty. If/when you are planning on disassembly, epoxy is typically not your first choice. But epoxy is a choice and, in some cases, it is arguably superior to other adhesives, including the animal protein glues, for specific lutherie tasks. Not all epoxies are created equal, though you can count on them all to form incredibly strong, waterproof bonds. Some epoxy formulations have excellent tonal characteristics where they, like hide glue, do not dampen the sound in any way but add a vitreous, glass-like component to it.
As has already been suggested, the best method for removing a bridge that has been attached with epoxy, or a bridge that has been attached with any other glue where tear-out is anticipated upon any attempt at removal, is to rout it off. By first securing the body, then attaching a jig, or fixture, to the soundboard that can cradle a plunge router, it is a trivial matter.
The fun part comes next, which is deciding what adhesive would/could/should be used to put the new bridge back on. Just think of the arguments that discussion can spawn amongst the right crowd(s)!
