Cannot switch from Pro to Light after evaluation period expired

I installed Directory Opus a while back and took advantage of the Pro trial. The Pro trial has now expired, but when I try to switch back to the Light version, it doesn't work. I get a pop-up message to click OK to restart DO, but the program never restarts and I remain stuck in the Pro version even after a machine restart. I have completely uninstalled Directory Opus and reinstalled it, but the certificate remains expired and I remain stuck in the Pro version. Any ideas?

Light and Pro both let you evaluate them for free for 30 days out of the box.

The Pro version (only) lets you extend the evaluation for a further 60 days after that, by requesting an evaluation certificate. (Pro has more features to explore, so we let people evaluate it for longer.)

So after the first 30 days you can no longer use Light for free, but can use Pro for longer for free via here: https://www.gpsoft.com.au/DScripts/evalrequest.asp

On the other hand, if you've already purchased a Light licence, you just need to install that into the licence manager; Opus will then switch to Light mode.