Question about compression

Does anybody know a compressor or a way to modify only the soft parts of a signal ? What I am looking for is a way to leave the signal above a certain threshold completely untouched and rise only the level of all soft parts. The problem with any compressor I know is that also all loud parts get modified…
Thanks in advance for any ideas.

I think Waves C1 can do upward compression. As does FLUX DExpander.