Regency doesn't have a list of what engine numbers correspond to what year, so you could possibly get away with it, as all RB26's have the same engine prefix followed by a unit number. Send in an application to modify, and they will tell you exactly what has to be done, and if it can be done, just write TBA where it says engine number, because that way they won't put anything to the rego of the car. So if you don't do it, it's no drama's.
There's a bit of a loop hole I found in the system to do with engine conversions when I did mine, but that's on a need to know basis