Let's say you run a small SaaS site, let's say it's a geoip lookup service, one of the more feasible type of small SaaS you can run by yourself. The features you may need are, say, a landing page, some documentation, a sign up and login process, accepting payments, and some admin management tool. All of above can reasonably be implemented in a JAM stack site. Things like the admin console page can have a lot of the page's structure statically displayed and dynamic data filled in from API calls.
IMO, "migrating" a web app from a traditional web app stack (LAMP, Rails, MERN, etc.) is an architectural redesign and depending on your existing codebase, could be a full rewrite. It's a paradigm shift. You may want to evaluate the cost of that vs the benefit.
HTTP GET /something/
I think JAMstack can do it, but if it involves looking something up in a database, writing something into a database, etc. that is out-of-scope.