The law allows various options (eg tagging piglets from holding of birth), but for practical purposes the following is best practice and covers legal requirements :
To slaughter - slapmark (both shoulders) or eartag with your herd no.
To another farm - under 12 months old - temporary paint mark
To another farm - over 12 months - eartag with your herd no or tattoo with your herd no.
A pig only needs id on movement, so whilst it remains on a holding it needs no additional ID.
So if was born on your holding, it needs no id at all as long as it remains there.
It arrived on a temporary mark, tag or tattoo, it can stay on your holding without additional ID, and if it loses a tag, this does not need to be replaced.
Any ID added by previous owners cannot legally be removed, so a sow sold 6 times could have 6 ear tags !
There are additional reqs for moves to shows and export.
So whatever you pigs have now doesn't need changing or adding to as long as you keep them. If they move, follow the rules above.