PHP Code:
USE SRO_VT_SHARD
UPDATE _RefObjItem SET MaxStack = 1000 where ID = (SELECT Link from _RefObjCommon where CodeName128 = 'ITEM_CODE_HERE')
Use this query to update MaxStack on DB side, and later you will need to edit them in Itemdata_***** manually. Just replace ITEM_CODE_HERE with your item code.