Hello there,
this is somewhat incorrect; we always advise starting a brand-new game each version as it reduces the risk of bugs to happen as well as to experience new content that gets added or modified in the early stages of the gameplay.
1.8 is a specific case and yes, no previous saves will work because the save system was changed with the release of 1.8, this change was advised from the beginning and is what allows the huge amount of save slots that are now available.