AFAIK, not possible with 7.2, unless you are willing to install from media again (and you happen to have the 220.127.116.11.0 install media).
18.104.22.168.0 had been out for 6 weeks and I planned to upgrade to it, but then they shipped 22.214.171.124.0 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 (126.96.36.199 rather than 188.8.131.52, 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 184.108.40.206
Using his example above, it would take you to 220.127.116.11 and not 18.104.22.168 -- 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 22.214.171.124 -- assuming it was the latest 7.3.1.x: