exec*() familiy replaces the whole process, keeping the process number (PID). If this is what you want...
One example inside Python interpreter. I replace the interpreter by echo. The first item in 'args' becomes the argv[0] which is the process name that is seen in ps or top.
>>> import os
>>> args=['process_name', 'bla', 'ble']
>>> os.execlp("/bin/echo", *args)
bla ble
/Users/epx $