So, if I understand it correctly, you're not talking about the account used for the AMS service (usually installed only on one machine - the management server), but for the avast managed clients AV service (deployed on each and every workstation), right?
If so, well, yeah, that's sort of a problem.
I mean, it has traditionally been a problem when a password for an account used to log on a service changes (this applies to many Windows distributed applications, not only avast).
Your best bet is indeed to re-roll the whole protection. That is, uninstall the currently installed management clients (using an Uninstallation task in ADNM) and then install again (using an Installation task in ADNM) - BTW both Installation and Uninstallation tasks can be found in the Deployment tasks folder in the console.
On a side note, I now recommend using the LocalSystem account for the avast services. It is functionally equivalent, and effectively removes this problem as the LocalSystem account never has a password.
Sorry for the hassle..
Thanks
Vlk