The only way to completely convert from one kind of grass to another is to kill and start over. I tired to overseed a KBG/tall fescue lawn with native grasses, figuring that over time, the KBG and tall fescue would die because they need so much more care than the natives. I had to overseed a number of times and, although the lawn now has native grass throughout, it still has a fair amount of KBG and tall fescue. If that's the case with grasses that have such differing needs in a desert environment, I doubt if you could succeed with grasses that have similar needs.

You could probably favor the tall fescue by fertilizing and watering more, since fine fescue needs less of both, but since tall fescue doesn't spread much, you'll probably need to overseed regularly. And even then, you're likely to end up with the tall fescue dominating, not completely replacing the fine fescue.

In our area, fine fescue is usually found only in shaded areas of the lawn - it's the major component of "shady lawn" mixes. It doesn't generally do too well for us - dessicates in the winter months, burns up in the summer.

So, I would say that one or two overseedings with a good quality turf type fescue mix will very soon come to predominate. Any relict fine fescue population would only be noticeable in the early spring - and then only as some finer textured turf.

Certainly, a September overseed would be a lot easier than a total renovation - and I think you will be satisfied with the results, overall.

brian