1)TREE NOT TO SMALL TO OVER WINTER OUTSIDE JUST PROTECT FROM RABITS THEY LOVER TO EAT SAPLINGS.
2A)go ahead & transplant this fall... soil should have some organics added & be well drained.
2B) it is very important that the soil be kept damp all winter long if ground thaws thenwater befor it frees again.
3) to remove from pot easest way is to cut the pot away from the soil ball.

I would make that organic matter about one cup of soil gathered from the floor of a well established woods. Not the leaves and nuts but the black dirt just under that.

The microorganisms that trees depend on are different species than the ones grass and gardens use.

thanks very much for the responses, good info

University research over the last few years has resulted in some changes in planting recommendations. Now they say to bare root your seedling grown in potting mix and plant in your native soil with no additives what ever. I have two apples and a pomegranate to plant this fall that I have grown in containers, and I will use this method myself for the first time. Al

Why do you want to grow this tree? What kind of seed did you start with? If you want apples, eventually, they may not be of high quality if you saved seed from a market apple. You must have compatible pollinators for any production.