Originally that was the plan Scotrail seemed to suggest. An odd service would still need units on it in each direction to balance the fleet out and assist with dmu maintenance schedules. I understand some of the current services that do that run with up to 3 units coupled together. It's probably also necessary to keep crews competent on both 158s and 170s as hst sets will still be subject to failures.
No depot has exclusive I7C diagrams, so it's not as if there was ever going to be a lack of competence on 158s/170.
So I'd be very surprised if any changes are required to keep crews competent, as well as to change the current arrangement of keeping all I7C services through Aberdeen which start/terminate at Perth/Dundee, DMU operated for rotating the fleets.
170/158 operated services served by HST trained depots include:
Tayside locals (Aberdeen, Dundee, Perth, Edinburgh, Glasgow)
Aberdeenshire locals (Aberdeen, Inverness, Perth, Dundee)
Fife Circle (Perth, Dundee, Edinburgh)
Anniesland (Glasgow only)
Invernessshire locals/FNL (Inverness only)
Correct me if I'm wrong on this but that's the general jist.
The new Tayside locals and Aberdeenshire locals at hourly frequencies pretty much off-sets what is lost from DMU workings being replaced by HSTs.