As V8 deeply broke syntax compatibility with the previous version, here are some notes I took when I switched my own installation.

Compilation

If facing some MQTT-related linking errors during compilation, upgrade paho library.

Global settings

ConnectionLostIsFatal - Doesn't exist anymore, as it's now the default behavior. I gave up implementing automatic reconnect; it's far better to handle it externally within the system's startup mechanism.

Modules' specifics

mod_alert

$notification= (and its alias $alert=) are deprecated. They are replaced by $namedNotification= which share the same syntax and usage.
Notez-bien : $alert (without trailing '=') declares an unnamed alert.
SMSUrl= and AlertCommand= are deprecated.
OSCmd= and RESTUrl= are no longer accepted as global directives. To achieve the same behavior, they must be part of a $alert section.

mod_1wire

Randomize renamed to RandomizeProbes