The International Certificate for Vaccination or Prophylaxis (ICVP) issued following vaccination is now for the 'life of person vaccinated' (see the example here) but previous certificates that might have expired should still be accepted by the receiving country. The WHO guidance on this is found here. I know a lot of people are under pressure from travellers for a further vaccine to be given to enable them to have a new ICVP because they've heard stories of receiving countries not accepting the expired certificate under the new guidance. Evidence of this practice happening within receiving countries needs to be collected. I would suggest if travellers obtain concrete evidence then you could e mail NaTHNaC about it at NaTHNaC@uclh.nhs.uk
The World Health Organization make it clear that existing certificates should be accepted by a receiving country. It may be useful for the traveller to carry the evidence (Annex 7 and Q&A) which is in the public domain obtained directly HERE (as already detailed above).
If a new certificate needs to be re-issued because for example the original has been lost, the detail from the previous record (where YF vaccine was given before 11 July 2016), should match the detail on the re-issued ICVP. The exception would be, in some circumstances, the UKYFVC centre number and the health professional detail. An ICVP written before July 2016 was 'valid' for 10 years and that detail should be replicated on a reissued certificate. You cannot put 'life of person vaccinated' on any ICVP where the vaccine was administered prior to 11th July 2016.