in another article here on SS, Emma (IIRC.... at least it was one of the cast members) said they have to reshoot the Kings Cross scene because when they shot it in the real location it was "very stressful and very busy". so chances are they're only going to reshoot the scenes they've already shot, not making the Epilogue any longer than whats originally in the script. and if they only called back Tom for one day, its probably something simple, like reshooting a line or walking to and fro.

And dont worry about his hair. Nearly all of them were wearing a wig at the Epilogue. Emma didnt sound to be bothered and she already cut off her hair when she had to reshoot