I think all of the Caps' 3rd liners are somewhat suspect. They do pretty well as long as they can keep the puck in the offensive zone, but in the defensive zone they are far from elite. Also, as a unit they bring very little offense.
The biggest improvement one can make, IMO, is at the position that carries the greatest load -- center. Then you have lots of options with Beagle, e.g. put him instead of Wilson if the latter gets in trouble with refs, or on the 4th line to help roll 4 legit lines. Also, a good center would be a useful backup in case Backstrom or Kuz get dinged up. And if you don't want to screw with chemistry, you can always start by using the new center with Bura on the 4th line, and maybe as a bonus get Bura to come alive.
Moving on to other things -- Kuz's line getting locked in its own zone is also a concern. Kuz's +/- probably means they generally succeed at keeping shots to the outside, but at the moment Kuz seems to need some (more) help. It's not really a knock on Mojo and Williams, because they are better at possession than Kuz, but it might take him a couple of more years to become stronger in his own zone... Just not sure how that line is gonna hold up when things get tighter and faster in playoffs.
One might note that Kuz played well in playoffs with Chimera and Ward.. so that's another potential benefit of getting a good center.. who might end up centering say Mojo and Williams, and Kuz might do better (on balance) with a couple of grindier types.
Just thoughts out loud..