There is only one way for friends with benefits to become something more and it requires becoming something less first. That is, your first must stop putting out for free. I’m not talking about free in the monetary sense, but requiring your old friend with benefits to start doing relationship stuff. Visits to your family on Thanksgiving, dinner and movie dates, doing laundry together, taking care of you after you get that root canal. That sort of stuff.
If the friend is willing to do those kinds of things, then resume having sex. At that point you are a couple. Presto, magic!