Can't edit PR and change merge branch if code is already merged

To reproduce:

• create PR with feature branch that shall be merge to branch A
• merge code to branch B
• edit PR and try to edit merge branch from branch A to branch B
• at bottom of page BB displays
  "No commits on feature-branch-name-here that aren't on merge-branch-here"
  and disables Update pull request button

Expected behaviour:

• I shall be able to edit PR and change in what branch code shall be merged even if code is already merged