Sites went down with "403 error" after changing PHP from 5.1 to any newer version globally on whole server.
Changes applied in control panel -> physical servers -> edit parameters option by cutting away a tick of php5 and ticking php 5.5.
After PHP globally changing on whole server, changes in customers' CP are required (choose a php55 module under web options and click "apply changes").
After that PHP will be changed but some websites could be affected due to the fact that code of the websites could not work with newer version of PHP.
To avoid this you could have two PHP enabled.