Tesco vouchers: trying to understand 'no change given for voucher overpayment'

Hello,

I've got some £15 Tesco vouchers which will expire shortly but I don't need them at the moment.

I read in the past that one could for example exchange each of them for a £0.50 restaurant discount and I would be issued with new vouchers which would be valid for the next 2 years.

Do you know if this can still be done?

I was reading on the Tesco site, eg:
https://secure.tesco.com/clubcard/caf-rouge/UK-009729.prd

and the small print reads:
*

There is no change given for voucher overpayment and the same voucher cannot be used across multiple transactions.

which seems to read that if you overpay for your voucher there won't be a new vouchers issued with the difference? or anyone has a link about where this rule is mentioned in their site (in case something goes wrong and we need to go back to them)?