Odds are that he won't. He would have more leverage within the Democrats than if he switched. He has been a lifelong Democrat with many friendships and other ties. But he wisely refuses to rule it out, preserving his leverage.
However, this could make Lieberman bolt:
Democratic leaders in the Senate vowed on Sunday to use their new Congressional majority to press for troop reductions in Iraq within a matter of months, stepping up pressure on the administration just as President Bush is to be interviewed by a bipartisan panel examining future strategy for the war. The Democrats — the incoming majority leader, Senator Harry Reid of Nevada; the incoming Armed Services Committee chairman, Senator Carl Levin...
A weaker American presence in Iraq would endanger Israel. I don't think Lieberman would stand for this.
Jack Kemp (not the politician) 11 13 06