Can I change command module?

Solution 1:

This feature has been added as of .20.

Editor Improvements

• The Editor no longer requires you to start with a command pod when building a ship.
• The Editor now allows deleting or replacing the first part of your ship.

KSP 0.20 Released!

Solution 2:

Try the Subassembly Saver Loader. Detach everything from the command pod and save those pieces. Then start a new design, select the command pod you want, then use the subassembly plugin to load back in the rest of the design.

Solution 3:

Note: this was valid for older versions, see ILMTitan's answer for the recent ksp version.

I'd take a look at Payloader:

http://forum.kerbalspaceprogram.com/showthread.php/29950

It lets you combine a payload and a launch vehicle. It works by reading the two craft files and creating a new one, therefore its very non-intrusive (imho).

Solution 4:

No it is not currently possible to swap out the command module you chose when starting your build, at least not using the options available to you in the game.

There are several threads on the Kerbal Space Program forums including a couple of feature requests for exactly this, although I can't provide you any links at the moment due to the KSP forums being down.

Here is a video on YouTube that walks you through editing your saved rockets to achieve a command module replacement in the current version of KSP: