Apologies again if this has already come up, but I've only just had the time to sit down and actually see what was occurring, rather than just being perplexed in the heat of the moment.
This has definitely been a glitch for a while, and I've just isolated it in 1.9.11 release as well.
When replacing a channels address with an address already associated to the dimmer part of a 2 part Channel (p1 Dimmer, p2 Scroller), the address moves correctly but REMOVES the dimmer part entirely from the source channel.
EG...
Chan 41 p1 - Dimmer - 2/18
Chan 41 p2 - Scroller - 3/4
Chan 42 p1 - Dimmer - 2/15
Chan 42 p2 - Scroller - 3/2
Scenario 1 -
41 [Part] 1 [@] [Enter]
41 [Part] 1 [@] 2/15 [Enter] - dimmer address from Chan 42
This entirely removes Part 1 (Dimmer) of Chan 42!
Undo Enter - Interestingly puts 42 back to normal but SWAPS the part numbers of Chan 41 so that Scroller is now Part 1 - Undo Enter again puts us back to the beginning.
Scenario 2 -
41 [Part] 1 {Replace} 2/15 [Enter] - Provides exactly the same results of removing the source channels dimmer part.
Scenario 3 -
41 [Part] 1 {Replace} 3/2 [Enter] - ie the Scroller address from chan 42.
This works as expected, leaving all parts intact but without an address for Chan 42's scroller part.
Can't see that this would be a 'feature', so I'm claiming it as a 'creature', unless informed otherwise!
Warren.