I'm just working on outsider's observation and intuition. Given that there have been official statements* that the whole fleet is to be
ERTMS-capable, and later that only a specific number will actually be fitted with the equipment, it stands to reason that fitting more with the relevant equipment should be reasonably straightforward. (I believe
@Gareth Marston ran the numbers and found that this subfleet won't give much of a capacity increase to the Cambrian services once the hourly timetable is fully implemented.)
There may not be a balise fitted to every non-ETCS units when they roll into service, but there will be an empty space where one can be fitted (and possibly a mounting bracket too).
*If you want me to, I can have a dig about for the sources and/or the relevant discussion threads at some point.