Sorry Maya, it is not possible to change your IP. You can only do this by Court Order or a specific resolution from your creditors, which is unlikely.
If you are able to fund the additional rent payment out of your overtime earned, your IP should make an adjustment when he asks for the 50% uplift. He will expect you to use your 50% first - but I regularly review things like this with my own clients.
I would not be looking for increased payments from you if your salary goes up, as your rent has also gone up. Make sure you keep a note of all additional expenditure, so this can be presented to your IP at the time of your annual review. You definately do not need a variation!
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk