Switched from BigSur to Monterey and as normal this pops out when trying to upgrade macports:
fmbp16:~ root# port -v update Error: Current platform "darwin 21" does not match expected platform "darwin 20" Error: If you upgraded your OS, please follow the migration instructions: https://trac.macports.org/wiki/Migration OS platform mismatch while executing "mportinit ui_options global_options global_variations" Error: /opt/local/bin/port: Failed to initialize MacPorts, OS platform mismatch
The error is explaining very well what is happening and the recommendation is to visit the Migration website for further instructions.
My recommendation (and tested of course) is to just download the latest macports, the Catalina version, install it, then proceed as normal with port selfupdate command.
Afterwards you need of course to run sudo port upgrade outdated and you may get another error, related to Xcode. But that is normal too and you need to update that one too (via App Store). Once you are done with that, run again sudo port upgrade outdated and will be OK this time.
If at the end you get too many compile errors and you cannot get out of it, just follow the migration instructions.
If you think those instructions are too long, you have to do the following:
fmbp:~ root# port -qv installed > myports.txt fmbp:~ root# port echo requested | cut -d ' ' -f 1 > requested.txt fmbp:~ root# port -f uninstall installed fmbp:~ root# port selfupdate fmbp:~ root# port clean all fmbp:~ root# curl -O https://svn.macports.org/repository/macports/contrib/restore_ports/restore_ports.tcl fmbp:~ root# chmod +x restore_ports.tcl fmbp:~ root# ./restore_ports.tcl myports.txt