They probably said it changed gender as they may have sexed them too early, then few months down the line well after a new owner has bought them the "female" turned out to be a "male" or vice versa. This is common (as I have found) as you may not find out an axolotls gender until it reaches its 18th month. Some of ours matured early 9-10months, very visible; while at least one of ours whom I thought was female was male - only spotted as he was quite happily laying his spermataphore all over the tank and trying to lead the ladies round! His cloaca is quite small compared to our other males; but is now noticeably larger than it was a few months back.
I also take what petshops say with a pinch of salt and discard it over my shoulder!