You are free to perform changes to the addon code as long as you never redistribute your modified version in any manner. The addon license always applies no matter if the addon is modified or not and shall therefore be followed as well. You are not allowed to perform changes to any systems of the addon that are used to control piracy. These systems can easily be identified by their weird-looking obfuscation. Making any type of changes to these systems may leave your downloaded addon copy unusable until the changes are reverted.
