The exception is covered in section 14 of the following guideline – https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/927349/explainer-for-part-2-citizens-rights-of-agreement-on-withdrawal-uk-ni-from-eu.pdf
I believe when someone asked about this there was no specific process for this, but you do need to keep records just in case you are questioned. See if Mike knows any different, but failing that you could check with the Austrian Government Brexit hotline:
The Federal Government has set up a special hotline for questions about Brexit: +43 (0) 800 222 666 (free of charge within Austria)
Operating hours: Monday to Friday, 8 am to 4 pm (weekdays)
Feel free to also contact us by email: service@bka.gv.at