Create a mapping like this so that you can press a key and it will call the function which converts the text for you.
:vmap b :call MakeMeBold()
Let's create a function now.
:let @b=substitute (@b,"a","A","g")
. :normal! gv"bp
The above code with the substitute function substitutes all
A for example. You can copy paste each normal and bold instead of
A in try code and use the same line multiple times with different values. A better approach will be to use arrays instead of multiple lines of code.
Put the above sentences in ~/.vimrc