I am sure that they have the power to change the terms of the probation if the need be.  But I am also pretty sure that you have the right to be heard when they do so.  It appears not that you refuse to do your community service but rather that it has been determined that you can not.  That is a good thing as failure to comply can come with it hefty penalties and fines.  I would call and speak with your defense attorney about the the possibility of having a hearing to modify the terms of your probation given your medical condition.  Your situation needs to be "humanized" before a Judge who can listen as to what you limitations financially and physically are.  But I would come up with a viable alternative for the Judge,  You need to complete probation some how and under some conditions.  Good luck.