I've been following this for the last couple of years and have come to the conclusion that there are no current plans to scrap any
EMU▸ /
DMU▸ at all, even 'Pacers', so the question is basically 'are a particular
TOC▸ 's new units additions or replacements, or a bit of both'?
So in the case of Thameslink, we expect their current trains, the 319s, to all go elsewhere - so in rough figures it looks like about 400 vehicles are replacements, and then 800 are extras. That's the one cascade everyone seems familiar with. But Thameslink also eventualy subsumes existing routes from Southern, Southeastern, and the
FCC▸ routes into Kings Cross. So in all those areas, there will be older EMUs available, perhaps for strengthening existing services of those TOCs.
Crossrail is the same, the 600 new vehicles are mostly providing new services, but a fair proportion of
NXEA▸ 's services into Liverpool St get taken over, so there's another possible load of spare EMUs - but in this case the existing fleet is quite old, so may indeed get scrapped.
You are obviously familiar with how new units for
LM▸ produces replacements for
FGW▸ 's ramshackle collection, which in turn allows eg
ATW▸ to get back up to full strength...
Getting back to your question about what's what on the rolling stock front, I find this guy's website about as useful as any, his fleet lists seem to be kept reasonably accurate.
http://www.thejunction.org.uk/ Paul