The official PC game(s) allowed you to switch on the fly, but then it had a lot of other little eccentricities in that version so it can't necessarily be seen as definitive.
If I were Zargon I'd say if they just found some, go ahead you can put it on now, but if they are carrying a lot of stuff and just want to switch I'd say wait until the next round. But rarely will you run into a real case of cheating, like somebody who wants to carry the Plate Armor and only put it on when they are attacked, or swap out the battle axe for the longsword and shield so they can always attack with Battle Axe but always defend with the Shield, negating the penalty effectively.
Or you could be really strict and say if you have both, you have to stow one for the whole quest and use the other.
But apart from the restrictions of these few equipments, there is no official limited carrying capacity or encumbrance (the Dave Morris novels limited the number of items you could carry but those were gamebooks and gave different amounts for the solo heroes vs. the group of heroes in Fellowship of Four).