Upgrading 365 Personal bought from Google Play

I previously bought OneDrive 365 Personal from the Google App Store and now want to upgrade to 365 Family purchased from Microsoft (to take advantage of the Workplace Discount Programme available from my employer). It seems that I can't do a simple upgrade however - Microsoft Support states If you bought Personal via Apple/Google Play, cancel it there first and wait for it to expire before purchasing the Family plan on the web. How can I do this without losing the 800gb of photos and files currently saved on OneDrive?