> are database tables structure changed?
> If they are as i can imagine, mail server will be unable to authenticate until i Upgrade ispconfig on them (I remember this when migrated by 3.4.3 to 22.214.171.124 and was changed password column or something similar)
No. The table which is used for authentication ahs not been changed.
when I upgrade cp, what happen to other VM? stop to sync until I upgrade them?
The servers of a cluster do not depend on the master for normal operation, you can even stop the master without affecting them. The only thing that you cant do unti the update is finsihed on all nodes is tao add new meilboxes or domains trough the master.
Btw. You should wait for the 126.96.36.199 release with the update as it contains bugfixes in the update procedure.