Yes, Mashima did recton Gajeel a little bit. All in all, I don't think it's
that serious of a retcon, I guess he thought of somthing either cool and/or funny in addition so he's fixing it before he gets deeper into the story.
Even when writing a story, not everything goes as planned on the brainstorming sheet. Let's see what he does with it (the motion sickness weakness) before we decide if the retcon is a good one or not. I still don't know why Laxus would get that way though...but it would explain him walking everywhere and using his lightning to teleport... And of course Cobra was riding on his flying snake (forgot her name) so that conveniently explains that one... I still don't like the idea of Wendy eventually getting motion sickness...
Still, knowing that even Sting gets that way...