How to change dos based prog?

Hello! Experts;

 I want to rewrite a dos-based program to build a window-based system.  
The old program was made by Turbo Pascal 6.0, and the new system will be made
by Borland Pascal 7.0.
 If you have experienced at this area, please let me know where I should
start.  I have a manual of Borland Pascal 7.0.