Replace Speedmaster Bezel with Smaller One?

I have the impression that modding is more a Seiko thing. Also, using a BASE 1000 bezel to "mod" a Speedmaster Pro sounds financially challenging, but if you happen to have one lying around, give it a shot, it's your watch. My guess is that the smaller bezel may not look right on the larger asymmetrical case.

If that first watch you show is yours, I would advise you take extreme care since the existing bezel alone is worth 5 figures. Imagine bending a $10-20K bezel removing it!

The mod you describe, putting the skinny Trilogy bezel (or a priceless 1950s original) on another Speedy is perfectly feasible, though not necessarily easy, so maybe leave it to an expert. Not all bezels snap onto all cases first time. There were several case makers for Speedmasters, at least 5 from memory and tiny differences do exist, even within a single manufacturer's models because production varied slightly over the years. Some combinations will be slack and some too tight.

ps the one you link to is not OEM ie fake. Are you happy to wear a mashup of genuine and fake parts? Promoting such things is frowned upon here.

pps the steel bezel you are referring to was actually used on the 2915-1 and 2915-2 and some -3 ie only the very first straight lugs, from then on the wider one with black infill was used.