sed -i 's/stretch/buster/g' /etc/apt/sources.list
I could equally well just use 'testing', but for some presumably deeply rooted psychological reasons, I like the codenames better.
For my veteran netbook mini, buster is now the 7th incarnation of Debian or Debian-derivatives in its 9 years of operation: Etch, Lenny, Squeeze, Wheezy, Jessie, Stretch, Buster. I'm sure it will also run Bullseye. ;)