I’ve always taught that if you have made significant changes to your book for one reason or another you should definitely consider re-pitching agents. I’m sure there are plenty who cringe when hearing me give that advice, but the truth is, what do you really have to lose? The one thing that always strikes me, however, when I read the comments to those types of posts is how pleased authors are, and that worries me just a little, tiny bit. What it makes me wonder is are you continuing to rework and re-pitch the same book over and over or have you moved on. I will tell you right now, if you don’t learn to move on, to write the next book and query the next book and write and query the next book after that you will never be published.
No agent and no publisher wants an author for only one book, and if you spend years reworking and re-pitching that same book you’re not making yourself a very marketable or publishable author.
So yes, while you can certainly re-pitch if you’ve done work, the key to success is forward momentum. Make sure you’re headed in the right direction.