I believe you are asking that if the agent has rejected your MS and then you revise it anyway, should you send it to them again? Is that correct?
For that I can't say for sure seeing as I am not an agent. But I can speculate, just don't take my word as law or anything. If an agent rejected a full, had a few suggestions but said they were going to pass, I think there was probably something fundamental about the MS that just didn't click with the agent. Otherwise I would assume they would ask for revisions and have it resubmitted. But it's tricky, and it would depend on your relationship with that agent up until that point. I also think if they have read your full, you could be within your rights to simply ask, "And if I make these changes would you be interested in seeing the MS again?" Of course if the agent says "No" it's important to simply say thank you and move on.
Hope this helps.