Here is an experimental tool to transform v1.0x data to v2.03:
You can one-off transform legacy data with this. Alternatively, you could run current data through the transformer, copy the generated link, and post that to the registry. Or the datastore could potentially use this to import v1.0x data.
Of course, if the v1.0x data is missing information that is required at v2.03, you’ll see an error, and the requisite fields will need to be populated. But it can still potentially save a bit of work.
Under the hood, this app uses an XSLT file to transform IATI v1.0x activity XML into IATI v2.03 activity XML (this one). Producing an XSLT file like this should probably be part of any major version upgrade.