We are all assuming that there will be a total tonnage limit to a treaty, we could have a building limit of no more than 1 or 2 capital ships a year with guns no more than 16" and total weight of 40k per ship.
Similar limits for CVs and cruisers construction will allow replacement of older units, but it will also allow for ships like the older Battlecruisers that have a hull form optimised for speed to remain relevant , with suitable upgrades.
With a slower pace of construction then refits will be a good way to keep steel armour rolling mills and gun pits open as part of the refit cycle, refitting guns to fire at 30° and refitting of fire control equipment to make the best of the improved turrets across the entire Battlefleet.
Refitting the 13.5" Battlecruisers as station flagships makes sense in this scenario, with oil fired boilers they will need less engineering staff, they have one less turret to man and the extra speed over one of the 13.5" battleships makes them more flexible in use.
Refitting the 12" Battlecruisers only makes sense to make the Kaisermarine adhere to the terms of the armistice, if they are reclassed as armoured cruisers then they might be worth the coin it will take but they will require extensive refits with bulging, New oil fired boilers, geared turbines and the main armament being thoroughly reworked.
If the cost of the refit can be done for less than the cost of an OTL County class cruiser then there might be some value in it as they should serve until the mid 30s at least.
The 12" and 13.5" Battlecruisers should have very clear instructions on what they should engage and what they should not engage, anything upto a Kongo should be ok but they need to run away from anything more than that. They need to be veiwed as cruiser killers and not as light battleships.