They can already. Everything Lumo could and would use, any and all diversionary routes, up to and including a reversal at Edinburgh, have already been done. Lumo won't be using electric for reasons other than the limits of electrification in Scotland.
In answer to what the next priorities are, it's up to Aberdeen/Kintore for BEMUs, followed by Dundee to Dunblane, Kilmarnock, and then the infill bits of Fife and the Borders. Inverness both ways will be a long way off yet, and it's unlikely that Dumfries to Carlisle will come before the 170s are life expired.
Basically, switch as much as possible onto BEMUs, then go along later and do the middle bits. BEMUs and part wiring is cheaper from a CAPEX standpoint, but EMUs and full wiring is cheaper from an OPEX standpoint, and DMUs are the most expensive units to run. So while the money keeps coming, the general trend will be to spread the up front capital costs over as long as possible, but reduce the running costs as quickly as possible by switching to BEMUs.