System migration compatibility with `DataModel`s about pf2e HOT 2 OPEN

I asked openly in the discord, but would #2 work if a data model exists for the document? @In3luki was under the impression that it will not because the source data will be altered by the data model, and that v12 has not changed in this regard. There was no clarification otherwise in dev-general.

EDIT: Clarified separately. By not supering up, we can avoid the butchering.

from pf2e.

Though we may wish to avoid it in the future, there may be a very strong perceived need for later asynchronous migrations. Should this be anticipated as inevitable and somehow accommodated?

This can only be addressed in core, but one advantage of sync migrations and duplicating the data we're overwriting to make that work is that we don't have to be afraid of future updates to the source data. This thing we did was inherently unsafe and its good that we have to avoid it.

One potential compromise that occurred to me though was flagging an item as "needs refresh". This could skip all later migrations and then asynchronously update the item afterwards. Skipping later migrations would avoid the problem scenario of updating an item to its most recent version, then running a migration that assumes an older version, which is a potential problem of our current structure.

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