Notepad++

Edit-Line Operations-Join Lines

What you would expect (remove line breaks)

Search and replace with regular experssions

Searchbox: ^(.*?)A; Find the first 'A'

Replacebox: \1B; Replace the 'A' with 'B' leave all in front if it untouched.

Where to edit commands in the run-menu

C:\Users\<username>\AppData\Roaming\Notepad++\shortcuts.xml