I would just let the plum flower and see if its fruiting and the plums are any good.
Sometimes,.....if it's a seedling it could be O.K.
If you're the only one within a block heaving a plum, then you should consider getting a pollinator tree
or graft on the same tree a branch or two.
But now you have two differant trees already.

Konrad

Probably the most common rootstock is Myrobolan. It produces a small refreshing juicy plum. You may want to keep it around until it fruits, and then decide if you like it. If not, try grafting it over to something else. If you fail on the graft you can try again next year.

Scott

It all depends on what you want. If you want to grow and eat your favorite plums, dig up the rootstock and plant good sized trees of what you like. If you have ten years to play around, wait and see what happens with the rootstock, try re-grafting it each year. You might get lucky.
I have a rootstock plum tree that has bloomed beautifully every year for 25 years, has spread sucker rootstock trees around the yard, but has never had fruit on it. I've tried grafting it many times, but- like you- none of my grafts took. I'm getting too old for this, might not be around in ten years. So now I'm replacing the rootstock trees with the largest trees I can afford.
Again, it all depends on what you want.