Think this has been answered in the British in Europe~Austria Facebook Group. Essentially you use the Article 50 forms and tick box 32. But in Vienna the Referat 5.2 has closed, so you go elsewhere:
MA35: https://www.britishinaustria.net/faq/referat-5-2-at-ma35-no-longer-exists-where-do-i-go-for-further-applications-to-ma35/
Forms: https://www.britishinaustria.net/application-forms-for-art-50-euv-residency-card-online-2020-12-28/
Guidance: https://www.britishinaustria.net/faq/i-have-my-5-year-card-and-am-now-eligible-for-a-10-year-card-can-i-apply-for-it-now/
If you have issues with the Ma35 saying you can’t do it until 3 months before the expiry date, ask them to double check with their superiors at the BMI.