I've not the rock-solid evidence to say they're staying, but no-one has the rock-solid evidence that they're going either. Words from a stakeholder is not enough. When I see the evidence from First themselves, via official means, I'll believe it.
Don't forget, the fast lines are to have bans on <125mph stock, if they haven't already. What are they going to run these Turbos on, the slow lines? Great, delay and slow down services...
Passengers love the 180s, they're a vast improvement on the service provision given by the 165s and 166s. 166202 was seen on Saturday on the 1523 to Oxford at Hereford, but that's down to engineering posessions.
So let's assume First get rid of all 14 180s to Scotland, Hull Trains or some other area. First do NOT have enough HSTs to cover all the services operated by 180s currently, nor do they have enough Turbos to cover them. Well, maybe with the tighest diagramming of any unit ever, but that's not sensible planning. Besides, Hanborough, as has already been mentioned by Angus (I believe that's his name), does not have grandfather rights allowing the HSTs to call at Hanborough. 180s can only do it due to having SDO and being shorter.
Let's also assume the 14 180s all go to Scotland. Upon what route could you seriously put them on? Glasgow to Edinburgh? Nah, too slow. Glasgow to Newcastle via Dumfries? Nope, slower still. Glasgow or Edinburgh to Inverness/Aberdeen? Potentially, but these routes already have the 170s from HT to serve these routes. I know of no reasonable route to use 125mph stock on. That's as prepostourus as putting 180s on Cardiff to Portsmouth Harbour services.
In any case, before I go on forever, my point is there is no logic in getting rid of the 180s. They were designed for use on the GWML, not in Scotland. Not on the ECML, not anywhere else. Paddington - Reading - Didcot Parkway is already packed out enough, to put Turbos on the 180 services wouldn't be sensible. They simply can't cope the same way the 180s can.