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White sapote has a long tap root. You'll need to put it in a larger/deeper pot for it to continue growing. I had some seedlings in gallon pots for 2 years and it stayed around 2-3 ft and thin. The same batch of seedlings in the grown grew 10ft and I had to cut it back to graft.If your weather is not too cold, you can repot and wait for the rootstock to push, and graft at any time like ScottR said.
I've done chip buds with White Sapote. The challenge is getting the buds to push after the buds heal. You have to do all the usual tricks, and I've had a hard time sometimes getting the buds to push because White Sapote is so vigorous that the rootstock will push growth on so many other buds, and even from suckers from the rootstock.