I've concluded that this can be a very tricky process.
Every time I've tried to repeat a procedure that worked before, it seemed to fail at first, the second time around.
In each case, I did finally get to my desired result. In my case, it was to join one of my GGMoms, already in my tree, with her probable sibling group, which was in another part of my tree.
My advice:
First, save a GEDCOM file of your starting tree so you can reload it, repeatedly, untill you get what you want done.
Then, play with these crucial options in FamilyEcho to get where you want to be:
1. Change or delete / Change parents.
2. Change or delete / Change partner.
3. Change or delete / Delete (so and so).
4. Add mother (to someone with only a father).
5. Add father (to someone with only a mother).
6. and, MOST IMPORTANT OF ALL, SAVE your changes after each step, before trying your next step.
All of this is very touchy-feely, but it's what worked for me.