Perhaps instead of transitioning legacy 5* to legend, which may be useless in many players rosters, you could instead make current legends ascendable to s-class status.
The s-class status of the ascendable 6* would be as simple as slapping 200-400 base stats and 10% on to rush and actives. Quickest, cheapest and most relevant fix to bridge the growing gap in player base.
What to do with 5*? Create new museum collections where they can be traded in for an ascendable 6*.
What do you all think?