it would be great if you could just drag a new instrument from the sound library onto an existing instrument (one that is in use in the pattern editor) and the existing instrument is instantly replaced by this new instrument.
that way you can loop your song or pattern and easily try out how the song sounds with another kick or snare or ...

currently dragging a new instrument to the pattern editor will add that instrument
maybe a 'Ctrl+drag' could be used for the replace action ? (or another modifier)

another idea : maybe 'Shift+drag' a new instrument on top of an existing instrument in the pattern editor could be used to 'clone' the pattern of the existing instrument and reuse this pattern for the new instrument