Re-designing a php application with the least amount of trouble

I have a current web application, and I'm embarrassingly it has not been designed in the best possible way, but, in my defense, it was my first app!

The problem is the application started of small, and the method used is for each object i have a file, but their not actually classes so:

/libs/product.php is a product

Within the product.php file i have something like:

function prod_new($name, $code) { //create product } ;

This is working quite nicely but I have to rely on other developers naming functions properly, for example every product function begins with prod_. Based on this logic, is there anyway of starting to use classes with minimal interruption to the end user?

For example, I was going to create /classes/product.php, and within the class do:

class product { public function product($name, $code){ return prod_new($name, $code) } } 

Then any new functionality can be designed correctly. Is there any problem with this other than worrying about replacing existing code. This would also lead on to unit testing etc...

Or am I making a big deal over nothing?

Thanks in advance!

EDIT: took out reserved word reference, thanks Gordon