Same question for me, even after reading about it here and there still I'm not 100% sure what exactly is being done to the bezel..? I think its about the bezel at least
The BLRO Bezel insert is slighlty lower than the BLNR or others I belive it to be due to the use of the ceramic to get the red which makes if marginally more brittle, so it sits a little bit lower in the bezel ring.
The lipmod is where you sahve a little off the inside of the bezel ring to effectively lower the insert, i think the same can be achieved by shaving the insert down but more likely to get broken.
Im trying to find out if the bezel rings on the gen BLNR and BLRO are infact the same size and would technically be swappable as a whole unit rather than just the inserts, which appear to be different hence the need for the bezel mod.
If the whole bezel ring including the insert and gasket were the same size apart from the top section, then swapping BLRO to BLNR or even GRNR would be pretty quick if using a bezel removal tool.
Granted the GMT hand colour would be different but I think if the BLRO was the base watch then the red GMT hand would work with the natve BLRO insert also the BLNR and the GRNR, where as the blue GMT might not look so good with the BLRO blue sections as they dont match but wouldnt look too bad with the Grrey dial and the Green GMT wold only really work with the Grey dial or Black.
To look at they look very close but on closer inspection the BLRO insert looks a little lower in the ring but I suspect that the internal diam is the same so both should seat over the crystal, maybe the BBNR might look liek the crystal is a little low perhaps if the insert is seat a little higher in the ring.