Can I change the transparency of a selection keeping the relative transparency each part already has proportional?

Each layer in GIMP can have the transparency for each pixel specified in two ways, that add to each other:

If you imported a PNG file with transparency, you background layer already has an alpha channel - you can keep that untouched - Just right click on the layerÅ› thumb on the Layers Dialog ctrl + L and pick Add Layer Mask - that way a mask is created. It behaves as a grayscale image, where black means transparency and white means opacity - you can select if you are painting on the actual layer or on its maks by clicking on the respective thumbnails - Just select the mask and paint it gray (by dragging and dropping a shade of gray on the canvas, or using the bucket fill) - and you have extra, prortional transparency for the whole image, without touching the original transparency levels.