How did you save them? Did you export as a jpg, png, tiff or other ...or save as a .xcf. The latter is the program's native image format and only with that can you make changes as it will save everthing you added except history. You cannot undo anything, that is part of the history. Once the session is finished there is no history anymore. Only guides, paths, selections, unmerged layers, layer modes etc. The layers still have the 'fx' next to them and they can be edited.
You cannot open this file in a lower version of gimp either.
You cannot open this file in a lower version of gimp either.
