I think you can't do that, since the 'Edit Simulation Command' window opens if you right click on a directive.
I would try and define two directives:
.tran etc.
.ac etc.
When you launch the simulation, LTSPICE will ask you which one you want to run. It will automatically comment the other one out.
Then replace the semicolon with a dot (two directives again). Close the plot, run the simulation again and choose the other one.