I wouldnt be optimistic on getting a top up to be brutally honest.
If the house was valuedat 470K a year ago, whats its realistic value now?
(-10, -15. -20%)
Taking worst case of -20% (extreme, but thats the market we are in), the house may be valued at 380K now. Thats not an inconsiderate LTV for a 270K loan in theory..However...
Switching is unlikely, most lenders just aint doing it. The fact that you have had arrears will show in a credit check and will immediately flash lights to alternative lender.
PTSB I dont think will entertain a top up based on your salaries. 66K combined does in no way justify a mortgage of 270K in the non boom rational lending environment.
Wish you the best