Thank you all for info. Will upgrade tonight.. Just to be sure.. right way to do this is just to replace 'lenny' with 'squeeze' in sources.list && apt-get update && apt-get dist-upgrade ? Or there is something else I have to pay attention on ?
I would retain the Lenny security update line in sources.list (for luck) until Squeeze is released as Stable, and start with safe-upgrade.
Good luck!