Can someone tell me what happens if I am in my 4th year of iva and I cannot get a re-mortgage of 85% of value of my home???? I know they can extend the payments to 6 years but what if I can't get a re-mortgage???

My mortgage period is coming to an end 1st Oct. and will go from 6.3% down to 2.5% giving me a savings of £500ish per month on interest only payments. What do I do...get new agreement for principal to pay same as before or tell IVA practitioner and give them the £500??? Tried re-mortgage back in Mar...