Newbie Question: Converting PHP3 files to PHP4?

Hi there, I have recently taken over the webmaster responsibility for an educational website. It is using PHP3 and MySQL. We had to move it to a new server, that as it turns out only supports PHP4. I have been looking through all the FAQs, resource sites, manuals and documentation, but all I can find as a solution is to change all my current file extensions .php3 to just .php ! Can anyone tell me if this will work before I start converting probably close to several hundred files?? I am going to change and test some files, but would like some advice on this! The type of errors that I get with the current file setup are when clicking on any of the links that lead to files in PHP3 are: You have started to download a file of type "application/x-httpd-php3", etc. Any other tips or info would be appreciated. This is a bit urgent, as I have just found out that the site "director/owner" wants it as functional as possible for the start of school this week... so no pressure really... lol. Thanks in advance. Michelle