For me most trouble was getting the java 17, but I found this page on the raspberry forum how to upgrade buster to bullseye (although officially not recommended). After that the upgrade to openHAB 4.01 was a piece of cake. Just had to install the old javascript engine for the old rules, change some rule code and fix (a lot of) units i the things. But that was all named in de OH 4 F.A.Q.
@lukics I was a bit afraid of the memory too, but after a day it all settled. Maybe (because of the reboot) it’s the raspberry file cache and JVM heap filling. After a day I observe the normale garbage collection saw tooth pattern and memory usage is stable again. Maybe it will happen for you too (hardware = rasperry PI and I configured the JVM heap at 1Gb). Timeline is a week, First 2 days messy because of upgrade, reboots and fixes on broken rules. After the last peak my fixes are complete and it runs normal again. It loses memory for about a day. And then the sawtooth starts and memory free stabalizes.
