Config file upload problem with Motorola SB5101E

I have set up a linux box with dhcp and tftp, and I am using docsis configurator to produce .bin files.
All of our modems have received this file, and everything works fine, but now I need to change configuration file to 2 of our modems. I need to change them so that they use different upstream port than the others.
Since they have our old config file in them, how do I tell them that they need to download a new config file ?

When our modems connect, they pick up address from dhcp, but they pick up a firmware file only the first time, and they do not ask for file anymore.

I have even tried to reboot modems, but they still keep old configs. Modems are Motorola SB-5101e.