You can do it, but be aware that the change will only be effective at the end of the billing cycle.
For example, let's say you are unlocking game Y. You decide that it is not worth unlocking anymore, and you want to change it with another app. You first need to remove game Y from your unlocked apps, and then start unlocking the app that is of interest to you. The unlocking of the new app is effective immediately. However, bear in mind that game Y will still count towards your number of unlocked apps until the end of the billing cycle.
If that seems unclear or you have questions, please contact us at: firstname.lastname@example.org