Hi
I would think that you payments will increase into your IVA although this may not happen until your annual review. If interest rates rise your IVA payments would be reduced by your IP, they have the authority to do this back down to your original repayment level and possibly further.
Regards
we are due the same thing in april next year plus a pay rise, and of course usual sorts of things going up. We will be likely to have an early review to sort it all out.