If its non magical - refiting it to another person in the same size catagory - the PHB
"Each suit of Full Plate must be individually fitted...resizing costs 200-800 gp "
it is the only aromr that mentions that its necessary - although there is room or HR about fitting other armors to oddly proprtioned people. There are other rules for non-humanoid armor.
If it is magical it should refit automatially within the size catagory. (ie elf to dwarf)
this is based on the rules for magic garmets on pg213 - and the division of armor and weaapons into size catagories.
If you are trying to refit it to another size catagory, and it is non magical - it cant be done offically.
If it magical and you are resizing to a new size catagory you need to add the sizeing power: +1 equv. (from arms and equipment I believe)