from a recent article in Rail magazine quotes the roll out plan to be for 2+4s to be introduced on the Edinburgh-Aberdeen. Once the 2+4 rebuild is complete the 2+5s replace them and the 2+4s start to cascade to Edinburgh-Inverness diagrams. Glasgow-Inverness & Aberdeen will use 2+4 once there are sufficient 2+5s on the Edinburgh-Aberdeen and 2+5s take over the Glasgow-Aberdeen once Queen Street platforms are full length. It is a gradual introduction of increased capacity.
Once the Aberdeen-Inverness upgrades are done 2+5s start on that route too, some as extensions from the central belt.
End game is Aberdeen sees 2+5s from Glasgow, Edinburgh and Inverness, with Inverness seeing 2+4 from the central belt. Always with the option to add an extra coach to formations should there be demand.