Depends on your usecase. If all you do is host a fileserver and run plex and maybe nextcloud then you don't need to migrate. Core is considered more stable then scale. In addition Scale removed the openvpn service in favor of running it inside an app, so if you're using the build in openvpn service on core you have to redo that setup on scale with an app.
If you want better support for newer hardware, faster developement (may also introduce new bugs/breaking changes), a better app system with more (and supported) apps go for scale. Keep in mind that scale currently has the 50% arc limit by default, but iX now considers it save to change it manually to more then 50%.
iX offers a sidegrade from core to scale, which includes configs and vm's. What can't be sidegraded are jails, you'd have to manuallly setup all your jails as apps. If a jail is not available as an, either in the official or truecharts catalogue, you'd have to try to manually create it by using the "custom app" feature.