Mitt Romney, more than anybody, would stand to gain from
portraying Thompson as having evolved. The more that Romney can
point to other candidates changing their positions, the easier it
will be for him to fend off the flip-flopper charges by claiming,
"Hey, everybody else does it!" Also, if this becomes a contest
about who can best pander to conservatives (regardless of prior
positions), that would seem to favor Romney, who has shown that he
has no qualms about saying whatever he needs to.
But keep in mind that while Thompson's views may have evolved on
a number of issues, he's been solid on gun rights in a field where
you have Giuliani who supported gun control measures as mayor of
New York City and Romney, who signed an assault weapons ban in
Massachusetts. As the campaign progresses, took for Team Fred to
hammer home the gun issue, because gun owners are a significant,
and very politically active, constituency that currently has no
major candidate to rally behind.