Dear Miss Snark,
My picture book came out two years ago. It got good reviews and won two awards. Sales were pretty good, but not sky-high. I'd like it to come out as a paperback or board book, but I don't know what steps to take to make that happen. My agent says to take it up with my editor; my editor says, "well, maybe..." Is there anything I can do, or is this my publisher's decision?
Your agent said for YOU to take this up with your editor?
Unless there's something vastly different about picture book deals than EVERY other kind of book deal, this is your agent's job. If she's not willing to do it, you need a new agent.
Look at your contract. See what rights the publisher bought. It's not a given that they bought hard/soft/board as a bundle. Probable, but not always. If they own the rights, there's not much you can do if they don't budge. That's why your agent should be doing this.