Stamp Duty liability ?



I'm in the process of buying a house that will become my PPR.

I've owned my existing PPR for 4 years 4 months.
I paid reduced stamp duty on my existing house as an owner-occupier. (Means own-occupy for 5 years)
If I want to let this property, what are my options ?

a). Let now, and pay the back the difference between the owner-occupier and investor stamp duty at time of purchase

b). Continue to pay the mortgage for 8 months, and then let without a stamp duty liability..

Would Revenue have a problem with b) above ?