Due to changes in Workflow module architecture between v6 and v7, it is recommended to Publish or reject all unpublished changes of product related documents before upgrading. Not doing so may lead to values from the last published version being saved to the latest document versions. This can cause issues if the SKU property value has changed since last published document version.
This is caused by the newly introduced SKU versioning feature in CMS v7. Where in CMS v6 the SKU field values were stored in versioning information for logging purposes, in CMS v7 they are actually used for by the versioning.