It is, Sean mixed them up.
Right click on Ashes in your games list, select properties, click on Betas and there'll be a drop down list with the different versions.
Yeah, not sure how I missed that - my only excuse being that it is usually GC3 that has had saves become incompatible with expansions.
That said, I was able to confirm that the new Ashes update indeed would have made old save incompatible.
Again, sorry for my confusion.
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