I’ve suggested before that although no-one has to exchange their Registro for the new TIE (that is a right guaranteed under the Brexit Withdrawal Agreement), the authorities would gradually make it increasingly awkward not to do so. One can understand the reasons easily enough: the Registro is a document for EU nationals which, technically, Brits were when they were issued with it but the UK is no longer an EU member state; and the TIE is an ID document which is perhaps preferable when they’re dealing with third-country nationals. And, perhaps above all, new British registrants are now only issued with a TIE because that is the established system for third-country nationals and getting all Brits transferred would contain the nationality within one cohesive and consistent policy.
I repeat, it is not a requirement, and the right to retain the Registro is guaranteed by the EU’s own co-signature of the Withdrawal Agreement … and since the EU signed it, EU member states like Spain are bound by it. Nonetheless the FCDO has issued this today:
Although it is not obligatory, both we and the Spanish authorities do now strongly encourage UK nationals who have a green certificate, to exchange this for a TIE. The process is: