Hmmm
Esentially what you save is a dicitonary.
The simplest way is changing the values in the state_dict before the new network loads them.
Something like
weights = net.state_dict()
weights['layer_name1'] = ...
weights['layer_nameN'] = ...
new_net.load_state_dict(weights)
To freeze certain layers: How the pytorch freeze network in some layers, only the rest of the training?