Does anyone know if its possible to replace a TY754 gearbox with a TY752 on an 03 WRX, will it go straight on?

 

It will fit but the 752 is not as good a box as the 754.
Also you must ensure you have the same diff ratio. If you have a TY754 VN you must replace with another VN gearbox. If you have a TY754 VB then it is more complicated and to check you need the full designator code for both boxes.
Putting a 752 in your car is not a good move.

 

Ok thank you for the info so will any TY754VN box be ok? what about the TY754VN1AA? as there's some on ebay at the moment

 

OK. You have a UK car with a TY754VN and you are replacing it with a TY754VN

 

Isn't the 1AA a different ratio than a 4AA though?

 

Your question was will it fit, not what were the ratios.
TY754 is a UK/European box. 3.9 front diff and 3.545 rear diff and 1.1:1 drop gear.
As to the actual gear ratios, look up a gearbox ratio chart.