It is quite a messy affair the whole "changing actors in the middle of the series" problem. At least OHMSS turned out to be great. Not so much for Diamonds are Forever, which I struggle to remember for anything other than the out of order affair. Seriously, if I had seen DaF before OHMSS, I wouldn't have even noticed any problem. DaF is also considerably sillier than the previous films, and I don't think it suits it well. Mostly because I don't find the supposedly goofy scenes that entertaining. I liked the process of Bond supplanting one of the henchement's identities and making the bond girl of the week he killed the real James Bond (which brings up the question, if he's so famous how come noone just shoots him on the spot? Not to mention he already faked his death already, so there should be more consequences of his name being out in the public). Otherwise... it's just mediocre. The locations are quite cheap, the scenes in Las Vegas are quite low profile and sometimes with very few people on the streets, the desert is just a desert, the moon landing facility made me chuckle, but it's otherwise silly (why did the stage vehicle have enough engine power and fuel to allow a full escape? Wouldn't that be dangerous and impractical for the shooting?), though the sea platform is quite good.
The plan is... ok? Though considering how powerful the diamond satellite turned out to be, why not just sell the invention to the highest bidder? He would have all of the riches and power he could want. And really, once Blofeld has supplanted the millionaire's identity completely, to the point of noone in the world noticing it, why not just stay that way? He would have all the money he could need forever and noone would've suspected a thing, though just him being so megalomaniac and egotist that he just wants to see everyone bow down to him. Maybe it's the spirit of this movies, but comparing it to Goldfinger's pragmatic, or the grounded scheme of From Russia With Love seem much more solid, simple and safe. Everytime Blofeld appears he has wackier and wackier plans that end up clashing with the more mysterious and threatening persona he built up in the previous movies. The most threatening thing he's done thus far movie-wise is murdering Bond's wife, which was more petty and much more personal scale than the rest of his schemes.
Overall, Diamonds are Forever is another "meh" movie, a 5/10. Maybe it's because I don't feel the older movies have aged that well, but considering we have standouts like Goldfinger, From Russia with Love and OHMSS, I'm not sure if that's really an excuse.
You know it deserves the GOTY.