Thanks.
So if I understand what you suggest, what I have to do is:

Load the old model.
Find the FrozenBatchNorm2d layers
Create a FrozenBatchNorm2d layers with PyTorch 1.7.1 with the old eps parameter.
Copy the old layers weights/biases in the new layers
Replace the old layers by the new one.

Am I correct?
If yes, I have two questions:

How do I replace one layer by another one?
How do I copy the weights and bias form one layer into the new one?