You can keep your lump sum but your pension will be abated if, when added together with your FTRS wage, you end up earning more than your basic salary in the regulars.
Abatement applies throughout your time in FTRS and will only ne removed when you retire. This post was all about those that have a pension in payment before 55 and how index-linking at 55 will occur and the cessation of commutation repayment.
I hope that helps?
LJ