So you'd be maybe looking at 85% mortgage to value. Have you looked up KBC mortgage rules and interest rates? Might be a negotiation ploy with EBS to get them to release you from the fixed rates. Not sure if they will play ball though, worth a shot though.
I see EBS variable for existing is 4.7%.
KBC is 4.56% for between 80% and 90% new business mortgages, but they have a nice 4.06 if you are at 60% to 80%, which you would be if you paid off the lump sum of say 40K (so 70%) and brought the loan to 203K versus value of 290K one year fixed available. You wouldn't have to even use all the lump sum to get that rate, 10K will do. So loan of 233, paying off 10K makes it 80%.
What you need to calculate is how much each mortgage would cost, and how much switching costs, and how much breaking the 10 year fixed would be and then seeing if it's worthwhile, bearing in mind that there are only 3 years to go for the end of the fixed rate.