Take note that changing the extention DON'T change anything in the file itself, just it's name.
Yes! Changing the format used in a video file REQUIRE you to use specific converters. Some are multi-format, some are single format.
Electro, May 2012
Yes, you will need to download special software, you can get it here:
http://convertflvtomp4.org
It is safe for your system and free for you to get, so you take no risk, and it converts the videos really quickly, too.