Here is the index that can get you to whatever "Card to ...." effect you can imagine:
http://www.conjuringarchive.com/tree.php?cat=774
For a signed card to lemon, you will need to learn the Mercury Card Fold, Bannon's Creased Lightning or Tommy Wonder's Two Second Card Fold and a billet switch or shuttle pass. Personally, I would use an orange because they are larger. PM me if you have questions.
There also is a great L&L DVD on Bill in Lemon that I think has a card to lemon effect. I don't remember if it was a signed card -- I'd have to watch it again. I remember that Doc Eason's Bill in Lemon was a signed bill.
One final note, the torn corner is a great convincer for lay audiences. It doesn't fool magicians, but it fools audiences. Heck, I do a card to impossible location with an unsigned and untorn card and the audience is convinced.