One month ago I updated one of my models with newer (improved) one. This was because I realised I made some mistakes on the old model. No buyer complained about the model but it still bothered me. I added new model to the same page and edited the description to explain the buyers which model should be used.
Now, if your first model was ok and your second model is even better, you can leave the old model and create new page for new model with different price.
If you also realized that the old model has some "mistakes" and you just want to offer improved version of it, then I see no reason for creating a new page.
After all, it's up to you.