Worth noting that, with this SRU, downgrading will still break, and there's pretty much nothing we can do about that. If you setup persistence on 14.04.3 or 15.10 or something and try to later boot it with <= 14.04.2, it's not going to work. I don't think that's a use-case we can even attempt to cater to, so we'll just have to accept that as a wart.
Worth noting that, with this SRU, downgrading will still break, and there's pretty much nothing we can do about that. If you setup persistence on 14.04.3 or 15.10 or something and try to later boot it with <= 14.04.2, it's not going to work. I don't think that's a use-case we can even attempt to cater to, so we'll just have to accept that as a wart.