Question regarding updating my model

AFAIK, if you update a model with the new files, everyone who has already purchased the model, will have access to those new files.

You could create a new model with the upgrades, and still have both up. This way the customer has a choice of a less expensive one of lower quality and a more expensive one of better quality.

"So if i were updated my improved model & it's price, let's say i updated it from 20$ to 30$, does the person who have bought my model can still download the updated model for free? or does he have to pay 10$ to download it?."

WOW!

Imagine you sell at 2 dollars and increase the price to 20.000 dollars!
Does the buyer have to pay you 19.998 dollars afterwards in order to RE-download some textures included within your model?

WOW!

Awesome!