Will not shift gears when engine is warmed up

Is the fluid lever okay?
Will it move in reverse?
These are the possible causes:
Trouble shooting, symptoms, vehicle does not move in forward gears or reverse.

Shift cable, control valves, fluid level, oil pump and torque converter.

Trouble shooting, symptoms, vehicle does not move in forward gears or reverse okay.

Control valves, forward clutch, one-way clutch (sprag) and forward clutch oil supply (blocked).

What do you mean by "transmission fluid level has not differed much."?
Especially the word "differed"?
Did you check the level?

Okay, but you just replaced the front seal which means you had to remove the torque converter, so if any fluid spilled out you would have to add some.
So, is the level full right now?

I suppose you replaced the front seal because it was leaking.
Chances are if you had a leak, at time you probably ran empty on fluid and might have worn clutch material, that dropped down into the pan and might now be actually clogging up the filter thus shutting off the fluid from being drawn in and pushed through by the pump.
If you remove the pan and see a lot of debris in the pan, either stuck to a magnet and/or clogging up the filter, change the filter and install a new pan gasket, and refill with fresh fluid once you get the pan back on and try that.
If it still doesn't move, let me know and if there was a lot of debris in the pan, also let me know that.