In most cases, making the switch to PHP5 will only require minimal, if any changes to scripts and/or PHP settings. There may be some very rare cases where your PHP scripts may not function or what may have been simply warning messages will actually cause errors in PHP5.
If you are using a web application such as Joomla, Dolphin, Drupal, etc., please first update to the latest version. If you are using custom written scripts, it is suggested to review the Changes from PHP4 to PHP5 and make alterations to your scripts.
If you must revert to PHP4 until you are able to ensure proper functionality of the site while you update your scripts, please Change the Default Version of PHP in the 1&1 Control Panel until you are able to adapt your website to PHP5.