No , don't touch the original tags. and don't add any, just record them in your Livestock record book as they are. Your movement papers will then tally with your records when you are inspected. The only time you will need to re-tag is if the original is irretrievably lost, then you have to replace it with a red tag with the original no ,which is of course in your record book.
Any animals born on your holding you will then tag with your herd/ holding no's. We got our record book free from the defra office , while the Welsh office charge £3:50p for the same book ! ....Daft ain,t it