Am I correct to assume that telephoto (and telephoto zoom) lenses do not necessarily need major design changes to be ported from EF mount to native RF mount?
They could do every EF lens with an RF mount tomorrow. However they wouldn't work any better than the EF lenses on an adapter.
Instead, the lenses they're releasing are significantly different than before, for the following reasons:
-- the RF mount has a far shorter "film to flange" distance. With no mirror, they can get the rear elements practically touching the sensor, when the models say that gives the best trade-off between contrast, sharpness, aberrations, distortion, bokeh, size, weight, and cost. With wide-angle lenses, SLRs needed "retrofocus" designs to have the lens project the image a minimum of 44mm or what have you to the far-away film or sensor. That mad the lenses less contrasty, less sharp, have more aberrations or distortion, and be bigger, heavier, and more expensive. So, for a start: any lens 50mm and wider, and maybe including 85mm, can be redesigned to simply be better on a mirrorless.
-- similarly, zooms apparently can be too, even if they're not wide-angles at one end. Note the RF 70-200/2.8IS seems to be half the size of the EF version.
-- fashions change. With smartphones doing all the mundane photography, people are only reaching for the interchangeable-lens outfits for the special projects. Before it was key to have a small 50/1.4 or 50/1.8 for every-day shooting. Even the 50/1.2 was designed too be small. In contrast the RF version seems to put size dead last on its list of things to prioritize: it's like twice the size of the EF version. And not because it's mirrorless, but because they're optimizing for a whole new level of sharpness and damn the resulting size.
-- technology improves. I'd guess the lens-design software improves in itself, and I'd guess that it uses monte caro and genetic techniques to find good formulas, and advances in this software advance the lenses they can supply. So do any new lens materials. So do any new techniques for making aspherical elements. Also, autofocus motors, better aperture diaphragms (curved blades, more blades) and IS sensors.