Read about a machine running Debian from 1993 (version Debian Linux 0.93R5) until today, running Bullseye (Debian 11, that’s what I’m typing on). It’s chiark, a webserver with some other tasks like mail, IRC, Usenet groups, and so on. Found on The Register.
So yes, you can upgrade Debian, no need to reinstall. I’ve done it myself several times. But still, congrats and thumbs-ups to former DPL (Debian project leader) Ian Jackson.