Thanks for the response Rolf but I am finding it hard to understand.
I am already getting 6.87% with NAB which will be matched by CBA. I find service provided by NAB is very bad. That is the reason I am thinking to move to CBA as they will be matching the rate plus giving around $1500 which i believe will be lesser than the cost of the move as NAB i believe as removed exit fees.
For me, the main reason I want to move to CBA is because I want to buy an IP very soon but it's very difficult to get contact with NAB banker & all my equity is with NAB (have got total of 3 properties now with LVR of 60% with loan amount of 785000)
Problem of keeping my existing loan with NAB and buying new property with new lender is that if I get the new property with same lender, I would have some equity & I will not have to forke insurance. Being not an expert in property investing, I fail to understand what value I would get if I buy new property with new lender.