One way is to have two sections, one read-only and the other not, each of which displays the property, and each has a visible-when clause to determine which you want to show. Then, for defaulting to "C", set the property's value to "C" before you display the section. /Eric
To add on to Eric's reply, another way out would be as follows incase your condition is not changing after the page is loaded.
I.e. the condition is set before the flowaction is loaded then with a pre-datatransform you can set the value to the default value... and also the when condition could be used to judge if it needs to be readonly or not. This will not need two sections.
However if the condition would be based on some other input on same screen then above reply by Eric would be good...with a need to refresh main section to display teh correct subsection of dropdown.
Another way out would be to circumstance your subsection for dropdown based on the property value (as per your condition).