https://letsencrypt.org/2020/09/17/new-root-and-intermediates.html mentions the introduction of new intermediate certificates, but it does not specify when the R3 and R4 intermediates will actually replace
the existing X3/X4 intermediates in the issuing pipeline.
For most domains, the switch would probably not matter. However, for torproject.org, switching to the
new chain will break website access for e.g. chrome users as there's a static pin set. (An update to the pinset is already in the pipeline [cf. https://chromium-review.googlesource.com/c/chromium/src/+/2427555].)
So I wonder when the switch happens, so we can get one last batch of 90 day certs with the old chain to give people more time to move to newer chromes (and firefoxes).