You can do this using the Append tool. Here is an example:arcpy.Append_management("C:/Temp/Python/Test.gdb/Centerlines", "C:/Temp/Python/Test.gdb/Streets", "NO_TEST", """NAME, #, #, C:/Temp/Python/Test.gdb/Centerlines, StreetName;\
Address, #, #, C:/Temp/Python/Test.gdb/Centerlines, Addr""")
In the above example, the field "NAME" in the Streets feature class is being populated by the field "StreetName" from the Centerlines feature class. Also, "Address" is being populated by "Addr".