I used byteball.fr/bb as my hub and applied the witness list change with MEJGDND55XNON7UU3ZKERJIZMMXJTVCV to replace JEDZYC2HMGDBIDQKG3XSTXUSHMCBK725. On top of that I replaced UENJPVZ7HVHM6QGVGT6MWOJGGRTUTJXQ with bittrex address QR542JXX7VJ5UJOZDKHTJCXAYWOATID2. Then I found I was not able to post any transactions due to 2 witness changes. Why the nodes are not able to find an earlier unit posted by MEJGDND55XNON7UU3ZKERJIZMMXJTVCV and see that my transaction is actually compatible with that one?
You can not change more than one witness from the default list. Limited freedom. Only one change is allowed for your unit witnesses list compared with any parent on the DAG. Oldest units on the DAG are not to be selected as parents in order to avoid a "flower" pattern and keep the DAG narrow as the time goes.
In other words the built in default witnesses list rules the world (centralization).