I'd like to say in advance that I've ridden the same bike for a long time and love riding but I'm not very knowledgeable about terminology or general trends. Apologies if I use the wrong words/phrases.

So I'm (unfortunately) moving back to Brooklyn after a long hiatus upstate during Covid. I was planning on taking my Dad's old Bridgestone MB-5 which I've used for years, but he had a sudden change of heart. Now I'm looking at other bikes on my local craigslist and there arent many good commuter/hybrids/mountain bike options, which is what I'm trying to find. The goal is to get something similar to the MB-5. There are, however, some decent road bikes. Two that are sticking out are a Centurion Comp TA (21" frame) and a Fuji 313 TE (23" frame). My general question is if it would be a bad idea to change the stem/handlebars to be a sitting position more like a commuter/hybrid. I'm 6'1" so although the 21" centurion would typically be too small for me in road bike terms, would it logically fit me if I changed the seating position to be more upright? or is that not how bike geometry works and I should just wait for something more along the lines of what I want? Obviously I would need to sit on them and see for myself, but they're pretty far away so I figured I would ask here first to see what you all thought.

Thanks in advance!