The easiest would be to ask the current owner to change his tags to double/EID ones and move him with those tag numbers. The current owner just has to put the changeover in her records book.
Abattoir may not take them (and really shouldn't do so) with single/slaughter tags once their big teeth have come out, as they are definitily older then a year.
You can also upgrade from single to EID Tags once on your holding, but I am never sure if DEFRA may just moan about this.... My guess is there are loads of lambs this year in the same situations.