Orange Tree Leaves turning Yellow- Why?

[Mikan]

I am currently growing a Japanese orange tree on my balcony. I just replanted it in a new pot with new soil and new fertilizer. BUT slightly before the transplant there were some yellowing leaves and currently there are still 4-6 leaves that are yellow.

Are these just old leaves I shouldn't worry about? Or is something worse at play now?









What do you think- is the tree ok and just adapting? Or is something else at work?