Hi,
actually it`s pretty easy.
If you want order number to be generated from numbers (for example 943751684) and not from letters (GFKELCTRY),
1. Open the file /classes/order/Order.php
2. Find the function generateReference() (row 1497 in version 1.6.0.9, for example)
3. Two rows below change the parameter 'NO_NUMERIC' to 'NUMERIC'
4. Optionaly change the number '9' to different one (it defines the length of order ID)
The whole row should looks like:
return strtoupper(Tools::passwdGen(9, 'NUMERIC'));
This calls the function passwdGen that you can find in /classes/Tools.php, if you want to play more with algorithm how the order ID is generated.
PS:
I`m not programmer and I have only basic knowledge of PHP.
I`m using Prestashop for 1 month only.
Still it`s possible to dive inside the Prestashop code and find the way how to change the behavior.