My /etc/apt/sources.list file contains the following lines:
1 ## Note, this file is written by cloud-init on first boot of an instance
2 ## modifications made here will not survive a re-bundle.
3 ## if you wish to make changes you can:
4 ## a.) add ‘aptpreservesources_list: true’ to /etc/cloud/cloud.cfg
5 ## or do the same in user-data
6 ## b.) add sources in /etc/apt/sources.list.d
7 ## c.) make changes to template file /etc/cloud/templates/sources.list.debian.tmpl
I do not know what is a re-bundle, but changes I have made to /etc/apt/sources.list have survived for years across many tens of reboots and several upgrades starting with Debian 9 (or possibly even 8).
First, get your buster system up-to-date and it may not be a bad idea to do an apt-get autoremove or equivalent, and reboot if necessary. Then…
At the most basic, changing “buster” to “bullseye” in /etc/apt/sources.list and also changing things that look like “debian-security bullseye/updates” to “debian-security bullseye-security” should prepare your apt sources.
Then run apt-get update, apt-get upgrade, and apt-get dist-upgrade or the apt or aptitude equivalents based on the program you like to use. Reboot.
You may also want to purge obsolete items. I have found that “aptitude seach ’~o’” can list those items (or at least many or most – supposedly, it is hard to know for sure), “aptitude purge ’~o’” will remove them.
It should go without saying to backup all important data off the droplet before proceeding. If you use do’s droplet backup utility, it may be a good idea to kick one off before starting this.
I have used the above steps with good success over several Debian upgrades across many systems – droplets and linodes, and also on machines in my home and office. Good luck!