I've had mine re-plated twice, (Both times were YEARS ago, through a music store) in spite of all the warnings about changes in sound, etc. As far as I can tell, the horn should be taken apart and all the openings plugged. Since that time, I've read cautionary advice about how specific you have to be with them as far as exactly what you want re-plated and what they will and won't do for you. An instrument repairman who has experience sending horns to these folks is usually aware of all the pitfalls, so if one does not know all the ins&outs, it might behoove to find one of these repair cats who has done it for a few satisfied customers and maybe pay a smidgen more, just for the peace of mind.
As for the sound, I've been told by a reputable pro (who also used to work for Dominic Calicchio and Zig Kanstul at Benge - you may have heard of him, LOL) who is a lot better player than me that my old 1980 37-bell Bach Strad actually "plays a lot bigger than most Bachs", and I've never been displeased with how she plays or sounds, at any point, nor did I or my teachers notice any significant "before/after" change in the sound that could be attributed to the re-plating jobs. My first teacher is of the opinion that horns will open up over time, especially if it belongs to a player that romps on it pretty regularly and plays with a lot of energy and air, and that has more to do with the sound than the plating/re-plating.
I once had a friend in school that jumped on the 'Marsalis/Monette-wannabe' bandwagon and had his horn stripped to raw brass, in an attempt to get the "darker sound" that was in-vogue at that time. I must say I was not a fan of the result, and he wasn't particularly good at maintaining the raw brass finish, either. As I return to the 'trumpet player conversation' after all these years, I find that the buzzword these days seems to be "commercial", which I personally find more appealing than all that fascination with "dark sounds" back then...
Now, if I wanted anything in "raw" metal, it would probably be a brushed-finish copper bell, but right now, I'm just really digging the full, bright "ring" in the sound that I'm getting from the same old silver-plated yellow-brass horn that I played for 14+ years before I had to all but give it up for about 20 or so.