How can we migrate projects from Madcap Flare (2021) to RoboHelp (2019/ 2020/ 2021)?
There is an old thread at Solved: Convert Flare 10 project to Robohelp 11 - Adobe Support Community - 7059990 but to the best of my knowledge it will still work. It may only convert to a Classic version that last shipped in RoboHelp 2019. There is no 2021 version.
It is not a perfect conversion but I tried it a year or two back with later Flare and RoboHelp projects and it did a pretty good job.
You will need to contact @johndaigle via the Private Message option on this forum.
The only other option I can think of would be to generate a CHM output from Flare and then see Reverse Engineering (grainge.org) on my site. I'm not sure how well that would work compare with John's script.
See www.grainge.org for free Authoring and RoboHelp Information
As Peter mentions, I'm happy to provide a Flare to RoboHelp Project Converter. If you'll please private message me, I'll send you a link for the download. I am not certain about the compatibility of the versions you mention, but it's easy to set up and try it out.