Don't worry, saves transfer and the only tale that actually changed gameplay-wise between v5 and v6 that was in v5 is Lyn/Rath/Kent's.
EDIT: Eliwood's changed slightly. There is a stair mechanic thingie, Marcus can get a semi-Brave Lance, and Natalie has a horse. As for the others the only other difference I can detect is Matthew having a Razor Sword instead of a Cutlass in Hector's tale. Just minor improvements. Lyn/Rath's was vastly improved, and even has an extra side-tale(Kent's).