The whole project can be converted, but need some extra manual work.
I have translated [1] the article from Polskie Centrum KiCad about the migration projects from legacy KiCad releases, and have got the permission from my redactor in chief to put it only on this forum thread.
Illustrations not been updated to English - there was no time.
MIGRATE PROJECTS TO THE NEW VERSION.pdf (1.7 MB)
(Original article can be found at http://kicad-centrum.pl/porady/przenoszenie-projektow-do-nowej-wersji)
The table which will help you fixing French layer names you can find at https://kicad.org/help/migration-from-old-stable/
[1] The translations from Polish to English isn’t my best skills so I use automated translation from time to time. Sorry.