I have already downgraded from Craft PRO to Craft Client and am wondering if I can downgrade again from Craft Client to Craft Personal as the project I'm working on does not need anything more.
Yes you can. If you click the Settings icon in Craft, at the bottom of the screen you will see a box that reads:
Upgrade Craft to take your site to the next level. Show me.
Click on that and a window will pop-up with the versions of Craft you can install. Yours will look like this:
Just click the Uninstall button and you will be downgraded to Craft Personal.