Consolidating my responses - thanks for your replies. Very interesting.
The previous owner installed a wood floor in the foyer which shortened the first step and the last (top) riser is taller than the rest due to the transition so by adding the 3/4 inch treads, it actually helped even things out. Our contractor suggested he could do it either way but I thought leaving the treads in place might be superior since the glue bond surface area may be superior to the bond of a replacement tread on the three stringers. Plus I don't know what was installed in terms of glue blocks under the stairs witihout pulling one up. However, I'm always open to more experienced perspectives so if there is more to share here, I appreciate any follow-up comments.
I was actually thinking of cutting a 2-4 in half, hinge it in the middle, and use it to apply gentle side pressure to the stringer (as needed) when pushing these boards up under the trim to the correct step. My neighbor weight in at 250 lbs. I'm not aware of any tool that could apply side pressure like that, but I bet they exist.