If you use the eu.playstation.com forums but haven't been on there recently it's worth checking these two threads:
Important Update: PSN Coming to the Forums!
PSN Authentication & Forum Changes – Update & FAQ
Basically if you want to keep ownership of your threads and posts you need to link up your current forum username with your PSN account using the steps described in the first link. If you do this your current forum username will be replaced with your PSN Online ID once the forum goes back online on the 20th. If you don't your current forum username will be locked out of use but you can still sign in with a PSN account, old or new.