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I've not used the migration option through the Azure portal, but that sounds correct to me - updating the resource IDs and subscription ID in the stack state.

Thanks for the answers! Little update, So I've tried to update all related stack files (backups, history, stacks) changing the old subid for the new one after the manual migration in the Portal. When I tried to change a property and do an update I got this error

updating urn:pulumi:myStack::myProject::azure:storage/account:Account::mystack: Error updating Azure Storage Account network_rules "mystack4cbe748": storage.AccountsClient#Update: Failure responding to request: StatusCode=404 -- Original Error: autorest/azure: Service returned an error. Status=404 Code="PatchResourceNotFound" Message="The resource '<https://management.azure.com/subscriptions/a5*******************************/resourceGroups/mystackmain229fd717/providers/Microsoft.Storage/storageAccounts/mystack4cbe748?api-version=2019-06-01>' was not found when performing the PATCH operation." Target="mystack4cbe748"

Either way, I won't be pursuing this method anymore since I've already started the destroy + update. I leave this here as insights for future issues.