I would have thought if they still on their original holding of birth, then you simply cut out the slaughter tag and replace with EID ones. Just get a batch of EID tags from the same place you bought your slaughter tags from (make sure it uses the same tagger/applicator), then just out them into your register as per usual.
I have so far always EID tagged ALL my lambs, it gives me more freedom to choose when to send them off or if i decide to keep them for flock replacements.
I had a really high ratio of female lambs to male ones this year, not sure why.