There is a difference between the converted LBs (like Willie or Tedy) who played DE in college and the tweener defensive lineman from other system like Jason Taylor who have the build of a LB but are used as speed rushers on the ends and therefore can play both roles with some success.
The 3-4 defensive linemen are big dudes designed to take on one or more 300+ pound lineman to spare lighter LBs from having to do so. The LBs may be 270 pounds because basic math (5 offensive linemen - 3 defensive linemen) says they may have to get around a lineman, but the weight differential is offset by the LB's running start and quickness. A slower defensive lineman would not reap that benefit and likely would not be quick enough to react to a play taking place further than the other side of a blocker. I certainly would not want to see a defensive lineman attempt pass coverage in the secondary. The Pats' defensive linemen will also show wear relatively quickly if running all over the field, which is pretty much what an LB has to do.
As such, the likely answer to missing LBs is a 4-3 scheme, not changing positions for true defensive lineman. Given Belichick's use of multi-purpose players, that is why defensive linemen have been used as fullbacks (an additional blocker with a little more quickness than the offensive linemen) and a LB, Vrabel, has been used like a tight end (able to shed blocks, run a basic route and get open).
