This drives me crazy. Today I wanted to remove 1 engine from a rocket and add a decoupler. Split craft, removed engine and added the decoupler. Replaced top section of craft. Completely ruined my staging. I thought that the staging for the section I had just moved out of the way would stay the same but adding the new decoupler seemed to send the staging into confusion. The only thing I find more frustrating is getting things to attach to radial decouplers!