Maybe I have misunderstood the question.
Yes you can upload the program, make the changes offline, and then download the changed program to the PLC.
It sounds like you dont have the original programs for the TP and/or the PLC. That is a very difficult situation if you want to make changes to the program.
It could be better to write completely new programs for both TP and PLC, but only if you have a complete understanding of the machine/proces and are capable PLC and HMI programmer.
Yes you can upload the program, make the changes offline, and then download the changed program to the PLC.
It sounds like you dont have the original programs for the TP and/or the PLC. That is a very difficult situation if you want to make changes to the program.
It could be better to write completely new programs for both TP and PLC, but only if you have a complete understanding of the machine/proces and are capable PLC and HMI programmer.