Hi,
I'm not sure if this will work, but try:
1) Loading the new model as an object
2) Pointing the Object make param of your population at the new object
3) Save the file
4) Quit TG2
5) Restart TG2 and open the file again.
I think that should cause the population to be set up with the new model.
You could probably also duplicate your existing population and do this, so you don't mess with it when you make the changes. Just disable it when you want to render the new one.
Regards,
Jo