Following Brexit, Who's a Resident?

I found this the other day, it's a two-year padrón renewal from the town hall for someone with an American passport. She owns property here in our pueblo, but she is still an 'extranjera no comunitaria sin autorización de residencia permanente'. A non-EU foreigner without authorization of permanent residence.
Now, residence give you rights. As EU citizens, we Brits don't need a work permit, we can vote in local elections, we don't need a visa and we have automatic rights to enter Spain (I have a visitor staying with me from Canada who, despite a two-week return ticket, was asked to provide either a receipt for a hotel booking or a confirmatory letter from me before being allowed entry at the Madrid airport).
But, we British residents have Residencia - some of us.  Those that don't will of course be out on a limb. Don't think that owning property will save you, any more than it saves the American above.
But, those that do, (and check your police green card) will find that we are, since the dreadful Interior Minister of Spain Rubalcaba in 2008 removed our tarjetas de residencia and gave us the dreadful passport/green-card combo, actually have a 'Certificado de Registro de Ciudadano de la Unión': a certificate of registry of an EU citizen.
But, following Brexit, we British will be in breach of the above.
Will the Spanish let it go, or will they offer to re-register us as 'Residents' after some extra formalities have been completed? Perhaps having a certain monthly income from abroad? Perhaps the 240,000 Britons registered on the padrón, and rather less Britons in possession of a proper residencia, will find that Brexit will be bringing some unwelcome change.