Replace mode is a tool that lets you carefully replace particles of one type with another, by simply drawing over the desired area. Particles of other types are simply left untouched.

Usage

HUD Replace Mode

Ctrl-Alt selection looks like this.

1. Press Insert to toggle the mode. Your HUD should reflect that the mode has been enabled.
2. Hold Ctrl+Alt and select an element to be the "old" type. This is the type of particle that will get replaced.
3. Select another element to be the "new" type. This is the type of particle that will replace the "old" type.
4. Draw over anywhere to replace any occurrence of the "old" type with the "new" type.
5. Press Insert to exit Replace mode.

Here's a video showing how the workflow looks like.