It seems I'm having a similar problem. I've gor a 900ss ally swing arm (in the 600ss frame) but the rear wheel is a later addition with a 25mm bearing (my spindle it 17mm).

Does anyone know if it's possible to just replace the wheel bearings from 25 down to 17?

Cheers

Yes you can the bearings are approx 2mm wider which isn't a prob you just have to adjust the external spacer's slightly. Its what I have done with mine.

As Phil says the answer is yes
replace the existing original fitment bearings which are

75162.2566 = 6005 2RS C3 47 X 25 X 12

with 6303 2RS C3 bearings which have the dimensions 47 X 17 X 14

http://cgi.ebay.co.uk/SKF-Rubber-Sea...item334a14c3ea

As Phil pointed out they are 2mm wider but have the same Outside Diameter and will fit the 17mm spindle