Persistence so far does not store/restore UoM but only the value.
I suggest to now forcefully convert a QuantityType item to its base unit before we persist it to statically work around our current lack to store the unit and to avoid having to refactor all of our persistence implementation.
Following up on #3121, now with #3143 merged we have default units for all item UoM types.

This added conversion will break some installations where users chose to apply units other than the one that science defined to be the base unit for a physical dimension. It's a philosophy discussion whether that's breakage or rather as I see it the opposite, overdue fixing of a long-standing problem, but either way it's a breaking change.
From a user pov it breaks e.g. their OH charts.

I suggest to make it an option so users can choose for themselves whether to apply it to their environment or not.
For details and discussion see #3121.