Can I resubmit a manuscript to a literary agent that has already rejected it, but after modifying it A LOT?

[Joss]

Like someone else said, every agent is different. Some will not reconsider no matter what, and some will reconsider as long as the manuscript has been completely overhauled. Just be upfront and explain that you submitted it before and have since overhauled the manuscript so that it's different than before.

A reason why some agents don't reconsider is because the writer changes small things and then resubmit and those small changes weren't the reason the manuscript got rejected - so the major reasons the manuscript got rejected are still there.

I'd do it. Just research the agent and read all their interviews, twitter feed, facebook, blog, if they have those things, and see if s/he mentions anything about resubmitting. If they say no resubmits then move on. There are plenty of agents in the sea.

Good luck.