I don't think it's completely clear that you won't be allowed to do it.
The Central Bank has not approved proposals for a product to allow people to trade up and bring their negative equity with them.
However, the bank might allow you to trade down.
If you have a cheap tracker, they would be enthusiastic about it, but then if you have a cheap tracker, you won't be reducing your repayments if you take out a variable rate mortgage.
Ask your bank and see what they say.
Brendan