ok, lets do the legal first
In England and Wales you come under the Pigs Records, Identification and Movement Order 2011, known as PRIMO 2011, Scotland has an equivalent law.
In England and Wales, a pig moving to slaughter comes under part 3 article 7
In Scotland it is just article 7
In both cases the wording is exactly the same :
7.—(1) No person may move a pig off a holding unless it has—
(a)an ear tag with the letters “UK” followed by the herdmark of the holding from which it is being moved; or.
(b)a tattoo showing that herdmark (with or without the letters “UK”).
Regulation 6(c) states that
(2) A tattoo must be applied either by tattoo forceps, in which case it must be on an ear, or by slap-marking equipment, in which case it must be on each shoulder.
So it needs either a tag or a slapmark of YOUR HERD NO., so the tag previously applied serves no use.
ok so that's the legal position.
A abattoir can impose any trading conditions it wants, and some will only accept pigs with slapmarks, so you do need to check.
In practise, abattoirs don't give a damn about whether the tag or slapmark matches your holding, they only care if the paperwork has the same number as the pig.
It is quite possible that you can send the pig with the current eartag, and it may be fine as long as the number is also on the paperwork. It is also possible that if the abattoir is having an inspection, that someone will spot the number is wrong, and you pig will be condemned. You would then pay the cost of slaughter, and then also abattoir charges for carcass disposal, and you don't get your pig back You could also be fined. All probably very unlikely, but I state this so that you know the risk.
If you tag the pig with a second tag with your holding no., you'll be legal, but probably confuse the abattoir, as they'll need to look at both tags to work out what the pig is. You cannot legally remove the existing tag.
Article 22 in England and Wales states " No person may, unless authorised by the Secretary of State, remove or deface an identification mark"
Article 16 in Scotland "No person may, unless authorised by the Scottish Ministers, remove or deface an eartag or tattoo applied under this Order. "
BUT if it 'falls out' then it does not need to be replaced, and you can put a new tag through the hole that is there.
However I'd suggest that you buy a slapmarker, and slapmark it. Slapmarkers are easier to use than eartaggers.