My personal view is that Nintendo will not reveal new hardware while their current platform is selling anywhere near 400k per week. There is really no precedent for Nintendo replacing an actively successful platform except for maybe when they replaced the GBA with the DS back when they were panicking about Sony entering the handheld market and giving Nintendo the PS2 treatment again. The Switch Pro is not going to just be a form-factor refresh like the New 3ds or the DSi, it's is potentially a major change in hardware meaning that is going to look more like a new generation from Nintendo's point of view.
I would be glad to be wrong about this because I think that it would be a good business move for Nintendo to go release a 4K Switch Pro in the middle of the hype for the Switch, but when has this company ever done business this way? My prediction is that we will get an announcement on a Switch pro model once the PS5 starts consistently outselling the Switch on a weekly basis.