insulating question

Greetings all from the great white north.I have an insulating question i would like to get some feedback on.I have a backsplit house 4/12 pitch just reshingled it last year.All the soffit venting and roof vents are working as they should.What i would like to do,to increase my R value is remove all the blown in fibreglass insulation then insulate with 3 inch polyiso between the trusses then blow the fibreglass back on top of it.A 6 mil poly vapour barrier is already in place(obviously under the drywall).My question is..am i setting myself up for a huge headache because im mixing the two different types of insulation?And am i going to create moisture by doing so?The reason for the polyiso is that it is no cost to me whatsoever.If it means anything i live about an hour east of Toronto and we do get some bone chilling weather here.Thanks.