2 points to mention-
1) Every sheep must by law have an EID tag, regardless of whether it is a breeding female/male or a slaughter lamb. These are always YELLOW if inserted at the holding of birth. Then a non EID secondary tag if the animal is over 12 mths old. Your lambs must have had EID tags as otherwise the abattoir would have kicked up?
2) The double tags must be matching- so if you were only supplied a lamb with the slaughter EID tag (with the flock number on the tag i.e. UK12345 and no individual number) then this must be replaced if keeping over 12 months of age with an EID tag and matching non EID tag. Both of these must be RED because you're replacing the tags on a holding that the animal was not born on. So you just cut out the old EID tag, record what is printed on it in your flock book, and then record the new tag number (holding and individual number e.g. UK12345 000123) next to it. Your flock book should have a page for tag replacement details and reason for replacement i.e. lost tags, worn tags, upgrading from slaughter tags to double breeding tags.
Sheep tagging legislation is hard to understand but once you've got your head round it it is simple. I had to get my head round it pretty quick when I took over the farm's Red Tractor assurance. This website below spells things out pretty simply. Disregard what your farmer friend has said as to be honest the number of farmers I've met who don't understand sheep tagging outweighs the number I've met who do !!!
http://www.fearing.co.uk/sheep-regulations-scotland