The WR50 will have completely different discs and wheels to the XT660R, also the XT is about 60kilos extra so the spokes will be taking alot more abuse than they would on the WR.

I would suggest you sell the WR wheels and get a set of XTX wheels for the R. Then its just a case of changing over the wheels and the R's front brake to the X (unless you want to start changing braking attachment points on fork legs or calipers etc)

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Plough on