It's YOUR book.
Firstly, you need to judge the scale of what they want changing. Is it small stuff? Or big changes? Will it impinge on the theme or tone of the book? Do you understand the reasoning behind the change?
It's YOUR book.
I made major plot changes to one of my books to make a US sale. I'm not sure that it's better - the change makes the tone different - but I understood why they wanted it changed, and it didn't impinge on the important stuff of theme and meaning. I was happy-ish to make the changes (and very happy to cash the cheques).
It's YOUR book.
But what if you're not yet at that stage, and it's an agent who is wanting changes. I think it still applies: you have to understand what they see as the problem that needs to be solved. If you don't understand what that is, you won't be able to re-write convincingly. You also have to believe and agree that the 'problem' needs solving. If you don't, you won't be able to re-write with any conviction. Re-writing without conviction leads to unsatisfactory writing, and disillusionment and demoralisation.
It's YOUR book.
Re-writing just to please someone else is doomed to failure. In my opinion, if you don't understand why they want changes or you don't agree with their reasons, then you have to question whether they are a good match for you. You have to write from the heart, or it's no good. Your agent really has to believe in you, or they won't be able to convey that to someone else.
It's YOUR book.
However, if you do understand their reasons for wanting changes and think they're good, then go ahead and re-write. In my experience, re-writing when the author understands the perceived problems means a stronger more confident second draft.
It's YOUR book.
The main thing is that if you don't understand, or don't like the direction you're being steered to, then don't re-write. You're not a puppet performing for others, and you don't have to be represented by anyone. Nowadays, you don't need an agent or a publisher to see your work made available around the world. And above all, never forget that it's not their book and you ultimately call the shots because - and don't you forget it -
It's YOUR book.