You cannot 'dynamically' change the mode an entity has been placed in. This is because all the static entities are saved together as the 'universe' file.
The scenario you are describing can be easily accomplished by doing this:
1) Place the first entity, and set its physics to 'off'. Set its main script to a destroy script.
2) Place the exact same entity in the same place. Set its 'spawn at start' value to no.
3) Create a trigger zone with main script plrinzoneactivateused.fpi.
4) Set the trigger zone IfUsed value to the name of the second entity.
Good luck!
C++ is labourless... if your name starts with 'L' and ends with 'ee'.
