if I wanted to fit a Rotor crankset, such as 3D+ or 3D30 can I just replace the cranks leaving the existing BB alone
If it's a 30mm axle then it should just go straight in. There should be a collet and spacers to take up the slack on the axle - it's longer than a true BB30 axle.
If it's a 30mm axle then it should just go straight in. There should be a collet and spacers to take up the slack on the axle - it's longer than a true BB30 axle.