How successful were the changes implemented on Animal Farm once Jones had been expelled from the farm?
This depends on whose point of view you take. For the pigs, the changes were wonderfully successful. They moved from just being another animal on the farm to being the leaders/controllers of the farm. At the end, although they do not look like human beings, the pigs "are" human beings.
From the other animals point of view, nothing has changed and there is no "success." The only thing that has changed for them is that they have new oppressors. In fact, their situation may be worse. Under Jones, their oppressor was of a different "species." Under the pigs, they know that their own are their oppressors, and this situation is probably less hopeful than their original situation.
This, of course, is the entire point of the book. If we allow it to happen, someone will come along and make promises that they will do things that will "help" us; if we follow them blindly, then we will be switching one oppressor for another. This is clearly what Marx/The Old Major had in mind in their philosophies.