The only way to update from Freya 0.3 to Loki 0.4 was reinstalling the whole OS.
With the new installer coming with Juno, is one of the planned features to give existing Loki 0.4 users the chance to smoothly update to Juno 5.0?
I think we'll be able to directly upgrade.
as you can see in the schema "Our OS section" (previous release means Loki for example)" we could do a clean install, custom, "upgrade"
however. we're still not sure if it's the new system that will be used. as they say in this article :
System76 is aiming to ship the new installer as part of the Pop!_OS 18.04 release in April. For elementary OS, we’d love to ship it in Juno (and are working toward that goal), but we do have a backup plan to ship the existing installer if we don’t feel the new one is ready yet. So, When It’s Ready™ as you might expect.