L.R.
Don't wait for it to "correct itself." Not all speech issues do, and her history of speech delays means she may be less likely to correct this over time on her own. The longer you wait, the more she has to "unlearn" a pattern and the more ingrained it may become. It's nice that you're trying to help her correct this, but a professional speech therapist who specializes in young children could help her correct it much faster and more accurately. Get her evaluated today. My daughter was an early and good, clear talker but by age four was using V for the TH sound -- saying "muvver" instead of "mother," for instance -- and it only took about four months of weekly speech therapy to take care of it. That was all. And she loved going, because the therapist specialized in kids and knew how to make it all seem like fantastic one-on-one play time with a very engaged adult.
And please don't wait for an elementary school teacher to correct it. They're not speech therapists and might not even notice the issue for a while -- teachers today are so overworked and classes so large that they cannot possibly work with kids on things like this. A teacher likely would just suggest you find a therapist, so you'd be right where you are now, only a few years further behind. I'd get her evaluated now and start some therapy to get her past it before she starts school.