There isn't a good reason to hurry, as far as I can tell.
I just checked, the official CRAN R-devel installer is still built with Rtools40, and Rtools42 is not available for download from the CRAN mirrors. Once there is a stable download URL for Rtools42 on CRAN, I'll add optional support for it at r-lib/actions.
Once R 4.2.0 is out, GitHub will hopefully include Rtools42 on their Windows images.
If you want to check your package with ucrt now, Rtools40 works fine for that, at least I haven't seen any cases where it didn't. If we find some cases then I'll add Rtools42 earlier.
Why do you think that? Rtools40 and Rtools42 actually use the same compiler version for ucrt builds.
Which is not the case for any suggested r-lib/actions configuration, btw: all Linux and macOS configurations use different compilers than CRAN.