Looking at a high-bypass engine:
As the LPT seals and airfoils deteriorate, N1 will decease over time when running at constant LPT inlet temperature (that's called EGT on many engine models, even though it's not the exhaust temperature). The N1 decrease means that the fan pumps less airflow, so fan thrust is DOWN a bit.
Conversely, the LPT is extracting less energy from the core stream, so the LPT exhaust out the core nozzle is a bit hotter; ergo core thrust is UP a bit.
So it's a tug-of-war; Which will win - core thrust or fan thust? My bet for most engines the total thrust will be down, but not much.
(NOTE where I have specified the EGT measurement point; it's sometimes called ITT or Inter-Turbine Temp. Not all engines are the same!)