Well, if you want a new room at the Imperial, then just mention you wanna get a room on floor 4 or 5 - these are the newly renovated floors. Avoid floor 2, which is not renovated yet.
Regarding the Bristol, a friend of mine recently stayed there (also SPG AMB) and he just got the basic room, because they were sold out. He left and went to another hotel. Reaching out to them is maybe the best chance of getting a pre-assigned upgrade to a newly renovated suite (Bristol suite). Proof me wrong, but I think the Bristol suites are the only newly renovated suites atm.