If you can graft the branch oto the seedling it will produce a plant from the graft virtualy identical to the valencia, the only differences being things like the speed of growth due to the dfferent rootstock.

Im not sure wether the graft would be compatible but i cant see why not, i have grafted calamondins from an old tree onto both lemon and calomondin seedlings

Incompatibility of top to root has to be rare in citrus because you never hear of it. Orange on a type of lemon (rough lemon) is common in Florida.

If your Valencia is dwarfed by being on trifoliate rootstock, you will lose the "dwarf" characteristic on your graft.

Cool, that's what I thought. I'm Ok with losing the "dwarf" characteristic, I'll just have keep it trimmed down.

I do have another question about grafting. My neighbors have a 15' orange tree in their yard that has sour oranges. Can I graft a branch off of my Valencia onto that to help with the sourness? Or would just the grafted branch have the sweet fruit on it?

Thanks again,
-Joe-

Just the new branch would be sweet