It is technically illegal to remove the tag under The Pigs (Records, Identification and Movement) Order 2011 part 5 article 22 :
"No person may, unless authorised by the Secretary of State, remove or deface an identification mark"
What you should do is either add a tag with your herd or slapmark. Both are best done just before they go off (as you plan to do), as there is no requirement under law to do so beforehand, and tags may get damaged.
As for your current tags, they are not hurting the pig I would leave them in, button tags normally are fine. It is flag tags that are more prone to being pulled out - Note: more prone doesn't mean they will, and we have sows with flag tags who are quite happy with them !