No, you cannot and should not remove the old ones.
Under the Pigs Records Identification and Movement Order 2007 Section 15
Removing or defacing an identification mark
15. No person may, unless authorised by the Secretary of State, remove or deface an ear tag or tattoo applied under this Order
Technically you should add new tags with your herd no. on, or slapmark the pigs.
In theory a pig over a year old can have any number of tags if it keeps getting sold. In practise this is rare.
The abattoir may look slightly askance at you when you arrive with your pig with two tags, and you would be good to then put both herd no.s on the AML2 eg in the ID column “tagged with current herd no. AB 12345, and previous herd no. CD54321”
Also tag in the same ear as the current one, as otherwise the abattior has a 50/50 chance that they have to look at both ears before finding the right tag.