Simply changing the extension may not do what you want - you will receive a cautionary warning to this effect.
Computers are pretty stupid - consider this FICTITIOUS scenario: You have a document written in english - let's call it 'letter.eng' - to tell the comp. it is in english.
You then change the extension to 'letter.frn', telling the comp to expect it to be written in french. Result - confused computer, as the computer is trying to decipher french, but is presented with code it doesn't recognise. (english)
You may be lucky, but be prepared to change the extension back.........
And I would suggest always having the extension on display - I've never understood Microsoft opting to hide them by default. They provide useful information.
DavidDude - please remember, we all started with minimal knowledge, and asking is the way to educate ourselves. Don't knock those who know less than we do. My suggestions are what I would try myself. If incorrect, I welcome corrections to my rather limited knowledge. Andy.