Many thanks to you !
SHA256 are the same. However I tried to replace as proposed above, but reloading apache2 resulting in an error.
So I followed this thread How to manually renew a certificate? - #3 by elmicha and this did the trick (as root) :
cd /root/letsencrypt/
./letsencrypt-auto --renew-by-default
I'll probably add this to cron so that I don't have to do it every 90 days.
Many thanks
Max