kebabking
Not a Girly Swot, but I like them....
Only Welsh people will ever call it by the new name. Everyone else will continue to call it the Brecon Beacons.
Not even sure it's that - my experience of Snowdonia under it's new name (Eryri), and of Snowdon (Yr Wyddfa) is that the only people who use the new/old names are people who speak Welsh as their first language, and so have been using the Welsh names anyway, and English 'incommers' who are desperate to be seen to fit in (people in outdoor gear shops, and artists, ime) - people in Wales for whom English is their first language, and English tourists, they aren't bothering.
Plenty of those who live in/around the NP aren't using the new/old names - and there's a term for forcing people to use a different place names/language because it suits a particular political/social outlook...