Switched from Catalina to BigSur and as normal this pops out when trying to upgrade macports:

fmbpro:~ florian$ sudo port -v selfupdate
Password:
Error: Current platform "darwin 20" does not match expected platform "darwin 19"
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Afterwards you need of course to run sudo port upgrade outdated and you may get another error, related to Xcode.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 Once you are done with that, run again sudo port upgrade outdated and will be OK this time.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