How hard is it to replace a bathtub?

Depends on your skill level.

I am replacing the walls in the bath area

Why and what kind of walls?

Is this just a soaking tub? Or is it a tub/shower setup? If it's just a
soaker tub (jacuzzi, etc) then you're only talking about dropping a tub
shell into a frame and handling the surface around it (usually tile). If
it's a full tub/shower then you're talking about dealing with a tub with a
lip around it and tiling down onto it. Again, depending on your skill level
it's not all that difficult. I mean, think about it, if the average nitwit
contractor can do it, how hard can it really be??

Doing it right, and dealing with the inevitable complications, is where
experience counts.