So far only a few 153s have actually been sold for scrap I believe, most are in (admittedly cold) storage.
Railways that bagged a Pacer probably won't also be in the market for a 153, and I'd probably expect that 156s will be more desirable than either 153/155s or 150s purely due to being more passenger-friendly (window alignment, easier for kids to see out of, etc.) though that does assume that any are in decent shape once their mainline career ends. It's also possible that 23m vehicles will be more awkward for a lot of heritage lines, as they'll not have needed to accommodate vehicles of that length before.