If Kerry's is appointed and confirmed to the State or Defense department within one or two months. A Special Election for the US Senate will take place March 2013. Governor Deval Patrick will either appoint a caretaker ie Paul Kirk or Barney Frank-so he can run for the seat himself in the 2013 Special Election or in 2014 Regular Election. Governor Deval Patrick appoints a person who is likely run and have a greater than 50-50 chance of winning in the 2013 Special Election or 2014 Regular Election ie Mike Capuano,Setti Warren, or Sonia Chang Diaz. Martha Coakley and or Tim Murray are likely to run for Governor in 2014.
In order for Brown-R to return to the US Senate- he must win the 2013 Special Election against a lackluster Democratic nominee. However in 2014-when he has to run for a full 6 year team. Brown-R will face a top tier Democratic challenger like Deval Patrick or Mike Capuano-D. In 2020-assuming Brown-R wins in 2014- will face Joe Kennedy the 3rd.
If Kerry's does not get selected to a Cabinet position. Brown-R needs to hope Kerry decides not to seek re-election in 2014 but Democrats will end up nominating a top tier candidate. Brown-R can challenge Warren-D in a 2018 rematch in which he loses by a double digit margin.

they have not served at least 2 full terms.

he could bow to Kochs.

He won his seat against, arguably, one of the worst run campaigns in the history of the Commonwealth. Add to that a low turnout and, presto, instant senator!
After spending two years touting his "bipartisanship", going do far as to use a photo of him and President Obama in one of his ads against Elizabeth Warren, he took a solid, if not overwhelming, thumping.
Whoever runs against him (if Kerry goes) will not sleepwalk through the campaign. And the party will proved to have learned its lesson.