I only tag pigs that are going to either the auction or the abattoir, the rest move between farms on coloured spray marks, this is perfectly acceptable. New owners will fit their own tags if or when required. When I used to buy weaners at the auction I would remove the tags and fit my own, as they had to have one of my own tags fitted to go to the abattoir.
The only pigs that would normally retain their original tags are pedigree breeding animals, as this has the unique number that matches the BPA herd book.