After the problem realized when trying to put up three astronauts for an extended stay in the Apollo 13 LEM, namely that the square CM CO₂ scrubbers didn't fit in the LEM's slots, were they redesigned?
No, the scrubber design was not modified.
The Apollo 14 press kit has a list of the changes made after Apollo 13:
The major changes to the command/service modules include adding a third cryogenic oxygen tank installed in a heretofore empty bay of the service module, addition of an auxilliary [sic] battery in the service module as a backup in case of fuel cell failure, removal of destratification fans in the cryogenic oxygen tanks and removal of thermostat switches from the oxygen tank heater circuits. Provision for stowage of an emergency five-gallon supply of drinking water has been added to the command module.
(and several more paragraphs)
The file doesn't mention any changes to the CO₂ scrubbers.
A thread on Yahoo claims no changes were made, but offers no references. One of the posters there does say he has an Apollo 16 LM CO₂ canister, and it looks identical to the Apollo 13 items.
The Lunar Module, LM 10 Through LM 14, Vehicle Familiarization Manual (large PDF) shows a drawing of the Environmental Control System (page 3-55/page 89 of the PDF). LM 10 flew with Apollo 15. The drawing still has round covers for the scrubber canisters, like the photo below.