is it possible with the plugin for auto install ?

No,
The plugin is not design to downgrade.

Did you try your use case on Demo ?
Is it working or no ?

I ve download the v7.4.2 version and fuse with projeqtor folder. Now when I'm logging on the app, i've message who want waiting for upgrade... is it long or is it blocked ?

Downgrading should not take more thant few seconds.
There is no db upgrade, so it is quick.

This is surely due to previous migration fail.
Just remove file projeqtor/files/cron/MIGRATION

hello, I downgrade to V7.4.2, it's works ! Now, I can create activity.

I have just a mistake now on V7.4.2, I have loose the picture of each ressources, maybe I need to copy a folder of the V7.4.3 backup, but I don't know which folder ?

Hi,
This conforms your migration process is not as expected.
Process is to overwrite existing code with new one, then reconnect as admin, and migration is seamless.
You can copy the directory (and of course backup db) before, but you must migrate original directory, not the copy.
Images are in attachments.

A patch is provided on thread : https://www.projeqtor.org/fr/forum-fr/2-submit-issues/9885-solved-empty-project-automaticaly-created-after-activity-creation