First of all, a question : is changing anything really necessary ?
What I mean is :
- It currently works OK, it seems
- Changing means :
- spending time changing the code
- spending time testing
- introduction of new bugs
So, unless you really have to... Is changing anything necessary ?
If your answer is "yes", and/or you really want to change something... Well ; first question you'll have to ask is :
- Am I going to develop a totally new version ?
- Or do I want to only update some parts, one at a time ?
In the first case, you will re-do the application from scratch -- which means you can do anything you want :
- MVC
- Framework, ORM
- re-think your DB schema
- re-think your front-end (JS Framework, Ajax, ... )
In the second case, you will enhance existing parts...
In this situation, what I generally do is use components to update the old parts of my application -- for instance, using Zend_Mail
instead of some hand-mande code (just an example).
Then, note that your question is opened to a wide range of ideas. A recommandation I could give would be to look to good/best-pratices, for both :
- development : MVC, Frameworks, ...
- testing : automated testings
- setting up some continuous-integration platform, like PHPUnderControl, for instance *(to run the tests automatically, analyse the code with PHP_CodeSniffer, generate the phpDoc, ...)*
All this can also be useful later, for a new project ;-)
And it's nice and useful things to know !