I did a round of querying in the fall that got a few solid nibbles, two partials and a full, and advice along the lines of "love the writing but this isn't as marketable as we'd like; make these changes and it might be." While I definitely intend to repitch to those three agents, I'd also like to requery a few others who gave encouraging rejections. What I'm wondering about is the new query letter. It feels weird to lead with a straight traditional pitch if I need to get the point across that this was a book they passed on that has undergone revisions and might deserve a second look.
If the agent has already read a partial or full you definitely need to let her know in the opening of your query that this is a re-query. Agents have short memories, sometimes, and it’s likely that if she already read part of the work she’s going to remember and feel somewhat confused. We do confuse easily. Was this something she saw before or does it just seem familiar? Make it very clear from the beginning what you’re doing.
I would simply start out with something along the lines of, “Back in December you had reviewed a partial of my book Oodles of Fun and while you declined at that time I have since done extensive revisions at the suggestion of other agents and feel the book is much stronger. I have great respect for your agency and am querying to see if you’d be interested in taking a second look at my work.”
Obviously, put that in your own words, but I think full disclosure is your best bet.