Usualy they are grafted because they are hybreds. They will still make a peach tree but they will most likley be different from the parent plant.

As Plantermunn said, since Red Haven is a hybrid peach, you'll get a tree with some of Red Haven's attributes, but it likely won't be "identical" to Red Haven. Generally, an own-root peach tree will grow more quickly and will shoot up taller, but that doesn't mean it will fruit any faster. The earliest I've ever heard of anyone getting a ripe peach from a pit-raised tree was 3 or 4 years. If you have room to grow the seedling peaches, it is a fun experiment.

I suppose you could graft your "Red Haven" F-2 seedling onto the root stock of your other tree that reverted back, but Scott or Randy or someone else who grafts will have to tell you how and when to do it.

Rootstocks are used for many reasons, but mostly to give your fruit trees earlier fruit production, improved fruit quality, and increased resistance to pests and diseases.

Most peach trees are self-pollinating, so pollination shouldn't be an issue no matter how you choose to proceed.