neontim,
I would have to say if it is not obvious how to dismantle the tool to replace the field winding I would suggest the time and effort to try would not justify the savings of DYI. Not to mention the safety factors of a tool which is used in an agressive envirionment. Jbartos has put it best Talk to Bosch or get a quote from the service agent. You'll need to get parts from them anyway.
I put a star on jbartos for highlighting the hidden dangers of trying to fix this type of tool
Regards Don