We never used the availability of a new simulator as an excuse to released paid updates, let alone force you to "purchase twice".
Some of our products which were released in FS9, like Zurich, or the original O'Hare, can still be used in P3D4.5 for FREE, because we tried to use whatever backward compatibility the new simulators offered to prevent asking for a paid update if wasn't strictly necessary.
That doesn't mean we just install into the sim and hope for the backward compatibility to help. More recent sceneries, like KSDF, KCLT or KMEM, even if they run in FSX too, offers different set of files made with the P3D SDK, to improve performances there. And still, we don't ask any money to upgrade to the latest version of the sim.
In some cases, like GSX, we made an huge update to support PBR, which wasn't "just" converting textures, but was redoing lots of the model from scratch, which tooks months of work, yet it was offered as a free update, so you could jump from FSX legacy graphic, to brand new PBR models in P3D 4.5, for free.
The only time we asked for paid updates was from JFK V1 to V2, which was considerably different and the update was very cheap, and when we offered to previous owners of the original KORD to get KORD V2, which was a scenery made entirely from scratch, not just a simulator update.
Nobody really knows exactly HOW MUCH work is required to convert ( or even do from scratch, for that matter ) a scenery for the next MSFS, and it's not even clear which sceneries will be WORTH converting, rather than redoing from scratch, information about the SDK hasn't been released yet.
We can only make an educated guess and, the main thing to note is that, a scenery that has been made from the ground up in PBR, like KORD V2 or LFSB/Basel, would not only be easier to convert, but it will also not look out of place in a 100% PBR engine. Same as GSX, which already is mostly PBR, expect a few models which we would have to update anyway.
Those will be surely be the first ones we'll work on to port over the next MSFS (in addition to a new one we decided not to announce right now), but while we already announced a 10$ update price for LFSB, it's too early to tell exactly what will be required to convert bigger products, like KORD V2 or GSX. We would like to offer free updates, when possible, or reasonably priced updates.
Under NO circumstances, you'll have to purchase twice, since even in the worse case scenario, which was KORD V2 REMADE FROM SCRATCH, we still offered a discount.