Actually you don't need to upgrade if it's 7mode only.
System Manager 3.x has two sets of binaries, one for 7mode and one for cDOT.
The Binary for 7mode is identical to 2.2, so the goodness of newer versions only applies to managiung cDOT.
There are no new features in SM3.1.2 over SM2.2 in 7mode. The only thing changed are possible security valurabilities and Java/browser support for the web server component.
Regards, Niels