Creditors not querying the 50% would be no guarantee that they don't notice 5years down the line and demamd the balance of the "missed " payments.
To vary the agreement will require a meeting. The IP cannot refuse, but if he is minded to he will simply recommend rejection, so it is better to get him on side.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014
A variation would at least provide clarity going forward and as Foggy says your IP must call a meeting within 21 days of your written request. If creditors refuse to allow you to keep any you could forego the bonus altogether and ask for a raise. This extra income could then be used to fund any increases in outgoings so would not be of benefit to creditors.
