1. When the plugin or theme update only concerns the correction of some errors, it is better to send only the update code that fixes the error and not the entire new version. The reason is obvious. By installing a new version, all possible modifications we make are unfortunately lost and we have to redo them from the beginning.

2. Furthermore, when the update only concerns a bug in the plugin or theme's operation, then sending the update with the error correction code should always be free, even if the free upgrade period has expired.

1. you should not modify original files. If your modification is unexpected and new updated version will not work, you will demand support over your modifications to get it corrected. Each update must consist of full set of changes to ensure integrity of product.

2. that's idea that would never work.