You will get a Basic Init Error if your constants (c_p1_x, etc) don't match up with your SERIN. Whether or not you can SHIFTOUT a new set of variables to the processor remains to be seen.
If you want some sample code that talks to the programming port, check out RoboCon at my website. The BSX file in there uses SERIN and SEROUT to communicate directly with a PC.
http://www.robbayer.com/software.html.