AFAIK, not possible with 7.2, unless you are willing to install from media again (and you happen to have the 126.96.36.199.0 install media).
188.8.131.52.0 had been out for 6 weeks and I planned to upgrade to it, but then they shipped 184.108.40.206.0 so I won't be doing that for at least a month.
In 7.3 there is new functionality that will allow you to upgrade to a specific version (220.127.116.11 rather than 18.104.22.168, for example), but I don't know how well it would play with "downgrading" to the previous version.
The OS has a lot of other software that the MWG software relies on. This too is updated when the appliance update is done.
As a result there is no easy (or supported) way to downgrade because not only the MWG software would need to be downgraded, but the other software as well.
The only "downgrade" would be a reimage, as btlyric mentioned.
To expand on what btlyric said about specific version upgrades, this can be done on 7.3+ by issuing a command like:
mwg-switch-repo -s 22.214.171.124
Using his example above, it would take you to 126.96.36.199 and not 188.8.131.52 -- assuming it was the latest 7.3.1.x.
Using the command below would revert you back to the 7.3.1.x, and if you did an upgrade it would take you to 184.108.40.206 -- assuming it was the latest 7.3.1.x: