Maybe it is different in Scotland but I was told that all sheep purchased on full EID must have their tags removed and your own red replacement tags put in. With both sets recorded in your stock book.
There are a couple of different ways to read this, and I want to make sure I've got it right. Are you saying that when you buy in tagged ewes, you have to remove their tags and replace them with red tags? That's definitely not correct, as the sheep's original number stays with it for life.
My understanding is that if bought in animals LOSE their tags, you should replace these either with red tags (new number) or with replicas (i.e. the same number as their original tag), which is then recorded in the flock book.
Also, if one of your own home-bred sheep loses its tags, it is permissible to replace these with the next number available from your own tags, provided you record this in the flock book.
That's my understanding for Scotland at any rate. Is England the same?