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I think if the insulation were cut as a snug fit, it would get stuck as you tried to push it up. Also there will be some variance in the gap between any two rafters. Either it will get stuck or you will have an air gap. If you can bite the bullet and pull the existing PB off, that's most of the mess out of the way in one session. I really would do this properly; we did ours to regs (then 5 inches deep) and the difference is phenominal. You'll thank yourself for doing it.
Cheers Richard