ok, let's spilt this into the two parts
Tags already in - legally you cannot remove these, The Pigs (Records, Identification and Movement) Order 2011
'22. No person may, unless authorised by the Secretary of State, remove or deface an identification mark'
However if they come out, then there is no requirement to replace ! So your might just 'come out' by themselves !
Next to go to slaughter legally they require either a SINGLE tag with your herd no. on it, or DOUBLE slapmarks on each shoulder same law The Pigs (Records, Identification and Movement) Order 2011
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”);
and section 6(2) (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 and must, in either case, be easy to read.