Congratulations! I think you've found a nice way to change party members without scripts. 
In any case, thank ya.
However, if you don't mind the use of "code snippets", I would suggest you use $game_party.actors.size == 4 on the conditional branch, under the 'script' condition. Clearly, the (4) could be changed according to the maximum size of your party.
The reason is that you could avoid using one variable for checking if the party's full, and use it for other purposes.
In any case, thank ya.