For what it's worth, you'll still be able to use the old roots for another two and a half years (until September 29, 2021), which is not quite five years from the date of that old post, but also way longer than half a year.
There are no configuration options of any kind in Google App Engine with managed SSL security enabled (since the point is to let Google manage it), which I'd rather keep enabled if feasible to avoid having to worry about renewals etc.
But I guess in the worst case I can just buy traditional certificates for a couple of years.