Convert these into a new "Scottish pounds" currency and then move onto the Euro from there.
"It's legal tender, pal!"
(Actually, it's not legal tender. The UK government accepts them, but they do not have the status of "legal tender". Never have done. But the point is that this doesn't actually matter - they've never technically been legal tender, but you've been able to merrily spend them up in Scotland all this time without fuss nor issue - because, of course, what ultimately matters is that you can exchange them for goods. That is, that people will happily take them off you. Which they'll do, so long as they're confident that others will happily take it off them in turn. And they will do that, because they're accepted by "the lender of last resort", who'll always take it off anyone, which de facto makes them a valid currency. In an independent Scotland, the Scottish government would just take over that role to give it that legal backstop to supply everyone with confidence to know they can spend them. They already have the mints printing off these variant notes anyway, it just needs the legal backing that an independent Scotland would have the power to directly do for itself.
And, ironically enough, they could actually finally confer on it the status of "legal tender, pal"... before they eventually dump it for the Euro instead.)
