Although the option to make an App is a Checkbox:
and you appear to have the option to unselect:
You will discover that when you go back into the App trial mode is still supported (ie deselecting the checkbox has not taken).
This is important as it is often proved more effective to have two apps – a free ‘Light’ version (ad supported if you are lucky enough to be a resident of the USA) with an option to upgrade to a paid version as a separate download.
NB I have reported this to MS as a bug