Swap RGBA channels around in Gimp?

The necessary tools are in the Colours > Components submenu:

• The "Channel Mixer" allows to assign linear combinations of all input channels to all output channels. This is the fastest way to swap e.g. red and blue:
  • Select red output channel, configure red to 0% and blue to 100%.
  • Select blue output channel, configure blue to 0% and red to 100%.
  • Apply with "OK".
• As Michael suggested, one may also decompose the channels and compose them again later, which may open additional possibilities (than just linear combinations).