Replacement windows are new windows. So what is your question?
It will be a lot of extra work and expense as you will have to remove or somehow get behind whatever type of exterior siding you currently have in order to maintain a good weather seal. You will also have to remove then replace the inside trim. If you plan to reside the whole house this may a good approach. If not, I would go with replacements. I just had Anderson replacements installed and couldn’t be happier. With the new narrow frame design you won’t loose any glass area.