They better not fuck this transition up, that should be Nintendo's no.1 priority at this point.
Because they don't have a great track record of transitioning from successful hardware platforms in the past.
We haven't heard a lot, but at the same time it seems like the entire chipset (lol) for the successor has already leaked out via firmware and Nvidia insiders (Tegra T239, codename Drake, indicated as NVN2 in Nintendo's firmware ... NVN1 being the designated abbreviation for the current Switch, so take a wild guess what NVN2 stands for).
We even know the number of CUDA cores the Tegra 239 has and can estimate performance from that.
My guess would be early 2024, they probably planned for earlier than that, but I think COVID pushed everything for everyone back a year. COVID shutting the world down (and office based companies) for a year+ is absolutely something that probably has to be factored in, you're talking about the largest global event since World War II.