We have 3 websites. All are running the free variations of T3 blank templates. The templates have not been updated since 2016. Now we find we need to update to higher PHP and the site goes down when we try it. Searching for others with the error suggests it's due to outdated templates.
How can we update these templates without losing customizations? We tried the extension manager plugin to update, however it didn't find any, even though some are shown on the website.