I cannot think why you would want to change to an NPPL right now unless it is to avoid the Class 2 medical. The PA28 is an EASA aircraft (as is the 172) so within the next two years (a transition period) you will not be able to fly an EASA aircraft on an NPPL. The licence which will allow you to do so is the new EASA LAPL(Aeroplanes)*. The CAA will start issuing these from the summer. You will also need an NPPL if you want to fly microlights (unless you hang onto your existing PPL - that will also allow you to do so). There will be a transition path from the old UK PPL(A) to the LAPL(A). If you have a currency issue you probably need to sort it out before 1st July.
*(LAPL = light aircraft pilots licence)