WordPress 3 has been out for quite a while now and they've already left the .0 version behind and are now at 3.1. For me, the release of 3.1 was a sign to re-evaluate a possible upgrade.

I've skipped version 3.0, because internal tests on my server have shown a significant drop in performance, especially script execution time. Now, with all the new features, a small drop in performance would be understand- and acceptable, but for me it was in the range of 40-50 milliseconds PHP execution time for every single page view - a bit too heavy for my taste.

3.1 (and possibly all the 3.0.x iterations before) have brought back some performance. It is still a tiny bit slower than 2.9, but it is now in an acceptable range and on the positive side, WP 3.1 puts less stress on the database and only slightly more load on the PHP side than my old version 2.9 did.

So it was time to upgrade my test installation, which is an exact copy of the live site to see if anything would go wrong and find possible problems with the (few) plugins I'm running.

Issues while upgrading from 2.9 to 3.1.1

The upgrade went smooth except one issue that caused my custom permalink structure (/category/pagename) to stop working. After some research I found one of the installed plugins to be the culprit. The multipage-toolkit plugin, which I had installed to allow publishing longer articles split into multiple pages, breaks custom permalink URLs. The fix is easy and is described here. After applying it, everything went back to normal and I found no further side effects. The (slightly) modified version of the multipage toolkit continues to work after the fix.