Change a keyboard layout for already typed text
I do not believe there is a native way to do this within Windows. However, there does appear to be a piece of software called Ochepyatka which allows you to do this.
From their website:
Using Ochepyatka you can convert text between different keyboard layouts within a second! Just select an incorrect text fragment and press appropriate keyboard shortcut - the program will make some magic and fix your text immediately!