Hmm, very interesting developments here today. So my former agent (who I sent my full, revised MS as she offered to read it) got back to me. Essentially, she felt the changes I'd made were really good ones, and was very positive about the MS as she had really enjoyed reading it, but there are things she isn't wholly convinced by. It was a long email and she explained exactly what there were. There was nothing I hugely disagreed with, and a few things I actually agreed with the moment I read them. At the end she said she'd definitely be interested in seeing it again if I did change things (though she also totally accepted if I didn't want to).
So essentially it's a R&R, but with no guarantee that she would want the MS even if I did revise it. How do I feel about the changes I'd have to make if I revised the novel along the lines of what she was suggesting? Pretty good, actually - initially I panicked because the big suggestions involve making a non-linear narrative linear, and structural changes are scary, but I've since worked out how this could be done quite easily. The changes would push the MS quite firmly away from thriller territory but that wouldn't be such a bad thing. The changes would also make the story simpler, which also wouldn't be a bad thing as there is a lot going on.
What I think I'm inclined to do is make those changes (ho ho, back to the revision cave) and see how I feel about them then. By the time I'm done, I'd hope that the new agent who has my full would have got back to me so I know where she stands (and perhaps also some of the other four agents who have my first three chapters might have got back to me by then, too). If they all blow out then there's nothing to be lost by sending the revised MS back to former agent again.
I feel weirdly nervous about all this, because I have a niggling suspicion I might actually end up with a better book by making the changes former agent has suggested, but new full agent has the other version... Argh. I'm aware this is not a bad situation at all, but I now really wish I'd only sent the MS to former agent and waited to see what she said rather than sending it to other agents too. Why am I always so impatient?