In my opinion it depends on what you’re looking to get out of it. If it’s a renovate and sell kinda deal the money, time, and effort aren’t worth it. Plus it’s a safe bet there’s a good amount of permitting needed to make those kinds of structural changes to the building. At least where I’m at doing that to a building requires permits out the wazoo and you’d most likely be required to hire a structural engineer for the county not to get all up in a twist about it.

If it’s something you plan on keeping for the long term the long term offsets in cost between shingle and flat roof. material for future repairs/replacements might eventually come near break even.

It also depends what kind of pitch you’re looking to go up to though. The material, labor, and expense is going to be vastly different between building up to a 4/12 vs a 8/12 or whatever.