I was thinking that you'd develop a plan to try and minimise the risk of cancellation. It could be that it takes months to fully implement over both lines, but if you focused on the Ascot/Guildford services first you could then train other staff up.
You also have to bear in mind that the 442's are currently requiring 2 drivers, with them off the network that gives you a bit more flexibility then you would otherwise have.
You could probably fairly safely bring in 1 unit in a matter of weeks, a few weeks later bring in a second and by October half term (circa 8 weeks) have the whole Ascot/Guildford services run by 707's.
While they are running services use them for staff training for the Netley line. Potentially allowing for a single unit to be replaced on those services mid November, with the full service running with 707's for the start of the New Year.
Each extra unit in service would then be an extra 450 available for other duties.
If it turns out that the 442's problems can be resolved then you just stop the 707 implementation plan. However if it can't be fixed or it's going to take longer than 2 months then you've eased the problems with the 442's not being in service.
Other potential options are 319's or 365
(assuming that you could even make then 3rd rail again), although they are going to take longer to deliver. However due to their higher top speed could be used across more of the network.
That could be phase 2, start planning on their introduction at Christmas with delivery of services starting to be introduced for the May timetable. As by Christmas it should be fairly safe to make a judgement of go/no go on future use of 442's based on if there's a fix and how long such a fix should take.