Richard Wigglesworth went off injured in the first half of a match I attended at Peterborough in mid September. On that occasion there was minimal faffing around. He took over as fourth official until half time when Josh Smith was hoicked out of the crowd to perform board duties.
They must have swapped comms kits, otherwise the replacement AR would have had to press to talk any time he wanted to speak to the referee. Unless there were just seconds left there is no way they would have done that.