I have stairs similar to these in the first picture with the closed stringer.
I want stairs with returned treads like in the second picture.
I have 20 steps, how much would it cost to transform my closed stringer to returned treads?
No carpenter or expert here but from what I see that looks like a total tear out & re-do - the existing stairs can not be transformed - they can't expand
you need new stringer, riser & steps
Pricing depends on where you live, what materials you would like etc...
Cost might also depend on how old your stairs are. If they don't meet current code, stairway will have to be extended (e.g., in our state, current risers must now be 1" shorter and treads 1" longer than they were previously). I was quoted $5K in 1989 to redo a 14-step stairway...needless to say, we stayed with the original, LOL!
You need an on site estimate. Your specific circumstances and finish product choice will determine cost.