Holy crap I finally found out, and I don't need a mapping.

`Vim` has the `&` command which just repeats the last substitution. The same way as `.` repeats the last command, I can move to the next `true` and simply hit `&` which will apply again the last substitution...Neat and simple.

I finally found out by googling

    vi repeat last substitution

which yielded as first result
https://stackoverflow.com/questions/382155/how-to-repeat-a-command-with-substitution-in-vim#:~:text=You%20simply%20hit%20the%20.,hit%20.%20to%20do%20it%20again.


Apologies.