Back in the old days - pre FADEC - ratings intermix was fairly common. To change the thrust rating of the engine you literally needed to replace the fuel control. So intermix was allowed, typically combined with specific "part power trim" procedures to minimize the resultant throttle stagger (and in those days, the 747 had a flight engineer who would make small throttle adjustments to line of the thrust settings).
Since the incorporation of FADEC, changing the engine rating is simple - basically changing a plug on the FADEC and relabeling the engine data plate - an hour or so job. Plus, on the 747-400 the resultant throttle stagger with rating intermix is a significant work load impact with a two crew flight deck. As a result Boeing has never certified ratings intermix on the 747-400, or any other model with FADEC engines (this is not to say it's never happened - it has - but it was never an approved configuration).
As eckhard noted, the "-T" variant of the RB211-524 is a performance improvement (better fuel burn, lower EGT) but doesn't change the engine thrust rating. "-T" and non-T engines can be freely intermixed, however you need to have -T engines across the wing before you can program the FMC to take advantage of the improved fuel burn.