Modified root php and classes php, what to do in the upgrade process

[manolo]

Hi,

In order to build my first Prestashop site I need to do some coding.
I have seen in the forum some tutorials that modify some root and classes php files; for example in...
http://www.prestashop.com/forums/viewthread/57590/developpement/tutorial__ajouter_un_champ_obligatoire_au_formulaire_dinscription

But before proceeding I'm wondering what is going to happen later when I upgrade Prestashop to a new release.
I have searched the web and the forum and I still have some doubts..

TPL files
As I can place a copy in themes/mytheme, I see no problem with the modification (as long as any smarty variable is changed/removed).

PHP files
For the PHP files belonging to the modules I see no problem (mainly if is a module developed by me that has no upgrade).
For the PHP files placed in root or in classes, such as classes/Customer.php, I'm afraid there is a problem (I suppose that the new version will overwrite my customized old version and I my changes will be lost).

Well I hope that some one can help me with this doubt or propose a cleaner solution for the php (such the one that exists for the tpl)

Thanks.