Modifying Existing Build definitions using the API

Problem description                                                    
We are using RTC of version 4.0.4.                                     
Current situation:                                                     
We have builddefinitions below with following paramters and properties :
-                                                                      
Case-I: There is a build definition for BUILD which takes four input   
properties                           
Case-II: One more build definition for DEPLOY which also takes six     
input properties                             
                                                                       
Desired situation:                                                     
Case-I: In this new situation, the build definition for BUILD accepts  
only two properties. Other two properties are removed since they are no
more necessary.                             
Case-II: In this new situation, the build defintion for DEPLOY accepts 
all six properties but the property variable type has been changed. 
                                                                       
How we can change the existing builddefinition properties to desired   
situation without impacting the endusers.                              
How this is possible using RTC API??   I would be nice if we have a codesnippet which modifies a build definition