Don't try to remove tags that are already in!
You would normally add a metal slaughter tag before they go - better to wait until they are actually loaded into the trailer on slaughter day if you can, or a couple of days before so they forget about it before you load them!
If you tag with a slaughter tag too early they run the risk of ripping them out - then it's all blood, panic and purple spray
Our abbatoir didn't mind who's tags were on, as long as the tag numbers matched the paperwork - a quick call to yours will clarify.
Hope that helps