Yeah, going to have to disagree with you. It would not be a different game, and I guarantee that people would still play other operators. It would definitely still be Siege, but then again people say that after every new op "omg 2 imba pls nerf game ruined ubi sucks ubi cant make games" etc etc etc.
However, there are a couple things I'd change about OP's idea.
1) Make it so you have to hit some kind of milestone to access the weapons on Veteran. For example, 100 kills with the C8 gives you the C8 for Veteran. That way there's still incentive to play the other operators.
2) Keep it recruit like in the sense that it doesn't really have a "special ability" like Jager's ADS or Buck's shotgun. He gets a frag and a breach.
3) The FMJ thing sounds like it would either be super pointless and unnoticeable, or OP as shit.
4) Make the uniform like the original Rainbow Six (like classic R6) ops. With the option of an elite skin.
5) If this was the idea, just like every other OP, only allow 1 per team. That way, you won't get 5 veterans vs 5 veterans.