Maybe not in one-go.
Try:
Add transparency (alpha channel) Layer -> Transparency -> Add alpha channel
Set a paintbrush to suitable size and put the mode to Color-erase set the FG to the color and paint over the area for replace.
Next, set the paint brush mode to Behind and the FG to the new colour, and paint over the erased area.
edit: Of course, if you are making an image from scratch in Gimp you should use as many layers as is necessary and then changing parts of the design is easy.
Try:
Add transparency (alpha channel) Layer -> Transparency -> Add alpha channel
Set a paintbrush to suitable size and put the mode to Color-erase set the FG to the color and paint over the area for replace.
Next, set the paint brush mode to Behind and the FG to the new colour, and paint over the erased area.
edit: Of course, if you are making an image from scratch in Gimp you should use as many layers as is necessary and then changing parts of the design is easy.