Change primary display through command line

I have not seen this. Two things I have tried that help me, however, are:

Pin windows that you want on the side monitor to all spaces

I've often treated my second monitor as more a static workspace that doesn't change no matter what space I'm in. You can do this by

System Preferences --> Expose and Spaces --> Spaces -->

Click "+" under Application Assignments, and add the applications you want to stay on the second monitor to "Every Space".

Run a virtual machine in the second window

Using VMWare I sometimes run Windows or Lion on the second monitor in full screen. Add that virtual machine to "Every Space" as described above, and then within that machine I can use spaces and have it run independently of the primary workspace.

Neither method gives you exactly what you want, but they provide a few other options you might try that will still give your working style a boost.

You can set a shortcut to move among your screens. In fact, the monitor you add is just considered as another Desktop space. So you can perform the following steps:

1. Open System Preferences > Keyboard > Shortcuts tab;
2. On the left menu, select Mission Control;
3. There will appear some options. First of all, check that the Mission Control function is enabled. There will be consequently shown some other sub-options, such as "Switch to Desktop X", corresponding a certain shorcut. The default one is ctrl+X, where X is the number of Desktop to switch to. You can customize this shortcut as you prefer.

Then, you can just perform the shortcut corresponding in switching to the Desktop space you want and it's done.

If you don't know/remember the 'identification X' of your screen, just show your Mission Control (in my case pressing F9) and check for the Desktop number.