I don’t know french, but understand your question so let me tell you what I did.
I did that transition last weekend and I had to solve minor issues. All in all, it was much easier (less troubled) than I had expected 
I updated my YNH, made a manual backup from webinterface (just ONE file with all base, configs and apps), copied that file to external usb/disc (just in case) and took off SD card. So a few hours without server, but your YNH is still there in case you need it.
I downloaded 64bit version, burned it in A DIFERENT NEW SD card, plugged into the rasPi and started the system.
DO NOT MAKE POST-INSTALL. Repeat: don’t do it.
connect through SSH (root/yunohost) and update the system with the latest packages apt update && upgrade; then copied backup file from old system in a folder (or just plug in and mount the usb where it was saved from old system; or put it in your PC and rsync/scp to the new one. Whatever, make it available to new YNH so you can launch a restore and use it.
using CLI yunohost tools backup restore /backup/file.... (do search for the actual command) restored almost perfectly my old system in the new.
My Problems, just examples that may not be in your personal situation:
I had some softlinks and folders in my nextcloud I had to recreate in the new filesystem, then backup restore went perfect.
one app that depends on a particulary binary (gotosocial) did not work untill I forced an upgrade and downloaded 64 bit version (files and config were recovered from backup)
gemini server lost configuration (easy to recover)
jellyfin also had reference to some folders that did not exist (so I created and then was restored)
On a side note I also moved system to SSD external disc.