No, Canon will not be doing just a conversion. These will be specifically designed for RF.
Not quite sure about that, the version III telephoto primes are less than 3 years old at this point, and they've really made them as light as possible. The focusing system already has fly-by-wire manual focusing just like the RF lenses.
We tend to not publish tear-downs of super-telephoto lenses. They're big and heavy, always well made and well engineered, like you would expect in something that costs as much as a used car. Big, heavy, and boring don't make for an interesting teardown. So we take them apart, make our in-house...
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I says similar USM motor as in the RF 50mm f/1.2L lens
And if they designed these with the rear elements closer to the mount, they would not be compatible with the RF extenders, even the 100-500 is not usable through the full zoom range when an extender is mounted.
So it would be a bit surprising to see brand new designs so soon instead of other lenses that are even older in the EF-mount, like the 300 2.8, but maybe the development was running in parallel with the version III EF lenses or something.
From the article: "Obviously, there is a business advantage in using the same subsystem in several lenses. But putting the same electronic focusing system in this EF lens makes me think that going forward Canon lenses may have a lot of internal similarity in either RF or EF mount. Converting the Canon 400mm f/2.8 to an RF wouldn’t be quite as simple as a different rear element and an RF bayonet mount, of course. The additional electronics for the RF Control Ring would have to be stuffed in here and probably some optical tweaks made, but the core structure could be very similar."