Could crews change 40 mm barrels at sea?

How easily could crews change Pom-Pom or Bofors barrels at sea?

Quite easily: ships carried spares, and the gun crew could normally change a barrel itself, with engineers helping when available.

Ships normally carried spare barrels for both weapons. Gun crews could change a barrel at sea quite quickly using those spares. Engineers could assist when available, but the crew could perform the replacement itself. Bofors barrels cost somewhat more, so their longer life also involved a higher replacement cost.