so you just want to change the last layer of vgg19 ?
you can do like this.
old_classifier = list(model_conv.classifier.children()) # get the classifier part alone
old_classifier.pop() # remove the last layer
old_classifier.append(nn.Linear(4096,2)) # add a new layer
model_conv.classifier = nn.Sequential(*old_classifier) # attach it to the original vgg model