Assuming that you have the usual clauses they can be reduced by up to 15% without a variation meeting, at the supervisor's discretion. Any more than that will require a creditors meeting. It could be that the IVA becomes unviable and you might have to look at other solutions, depending on alll of your circumstances.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk