Do further developments on a patented product void the patent?

First of all, there is no such thing as a provisional patent, only a provisional patent application which must be converted into a regular non-provisional patent application within one year of its filing date or it simply dies with no rights having been obtained. At the time of filing the non-provisional patent application, you could add the improvements.  However, you should probably consider registering the code with the Copyright Office. I suggest that you contact an intellectual property law attorney who can review everything and advise you how best to proceed.