Sure, guys get thrown out of position midseason all the time due to emergency.
But when your LT is down most of preseason, and especially when it is not certain he'll be ready for first game, whoever is a candidate to play LT will get some game time there. The Packers are clearly confident that Barclay is improving and will be the LT backup, despite overwhelming fan opinion that Bulaga should go there. And McGinn opinion that Walker should be the guy.