Mid-life overhaul.
The 350/2s had it as well because Porterbrook wanted the units returned in an acceptable condition when the 730s come on stream. The 350/1s, now approaching 15 years of age, were due to be next in line.
It just so happens that the work being undertaken at Long Marston is for corrosion repair, and (for the /1s) removal of the old LM vinyls before any metalwork repair, before a full repaint in undercoat prior to returning to Northampton.
If traffic demand requires it, the unit can be pressed straight into service in this "ghostly" livery (see 350123 as an example), but when its time comes to receive an interior refresh/refurb, it gets dispatched to Bletchley. I'd imagine new LNWR vinyls will be applied there too.
By contrast, the Southeastern fleet of 375s had their midlifes done at Litchurch Lane AFAIK (which makes sense, as that was where they were built) as one package (corrosion repair, refurbishment, repaint).