What Andy says is true. Changing commas and points through the OPTIONS ADVANCED, fully messes up the system

Your clickbait in the heading says "convert" and what you go on to describe is something totally different from conversion.
In fact, after carrying out the steps you describe all the commas and dots in the cells stay exactly the same as before,and you have accomplished NOTHING resembling conversion.
What your steps do, is force Excel to interpret the contents in the cells different, as if dots and commas were the wrong way around.
And what REALLY sucks with your brainless advice here, is that the poor idiot who might be stupid enough and believe you, and actuallytake the action you recommend, will find that none of their other excel files work anymore as excel will interpret dot and commathe other way around in each and every file they open, so they might have worked around their difficulty for that one file,but the other 500 excel files on their hard drive were broken by your irresponsible idiot advice. Sheeweeeshhhhh