One issue that concerns me is that Plesk installs and configures all the software required during its initial setup but there is no documentation about what can and cant be changed / upgraded / updated.

I presume, unlike other CP's (cpanel) it doesnt keep the software it installs updated.

So far, Ive been very reluctant to make any changes to anything on the plesk server.

Are there any guidlines on what can be safely upgraded without breaking plesk ?

I know for example that mailenable pro cant be used. Is this the same for newer versions of php etc too ?

 

Execpt the 'internal ' or build in PHP, you can update everything.

We updated apache, MYSQL, bind and other stuff without any problems.

Be aware, that the original setup of a software might change pathes and others. This could kill plesk. Keep an eye on the registry ;-)