First you don’t really rename an export, you are changing the directory name that is exported. Now an NFS export on Isilon can contain multiple paths in the same export. So I guess the answer is that yes there will be disruption, but that’s mainly because the clients must:
1. Change their automount / fstab entries
2. Shutdown the application writing to the mount
3. unmount the current mount.
4. You make your change; so first (mv /ifs/data/mqs /ifs/data/mgx), then isi nfs export modify <5 or whatever the ID number is> –path=/ifs/datamgx
5. They re-mount the export
6. They restart the application and ensure everything is working.
All in all if done properly you could probably do this with maybe 5 minutes of downtime. You could try and cheat and give the export an NFS export alias, but really I would just keep it simple and do it properly. Your mv command is only updating a single node (that of the directory in question), so none of this should take a lot of time.
Just my approach, however, others may have other thoughts.