A (blacksmith) sees a girl who pleases him, and sets out to sleep with her. She tries to foil him with magic transformations, but he proves as sorcerous as she, and gains her maidenhead
Sharp bowdlerizes "gain my maidenhead" to "change my maiden name" (!) -PJS
Bronson believes that the ballad "Hares on the Mountain" is a very-much-worn-down version of this piece. This is, at best, currently beyond proof; personally, I don't believe it.
The idea of gaining a lover who is changing shape has ancient roots. We find it in Ovid's "Metamorphoses," where Peleus (the father of Achilles) finds Thetis in a cave and attempts to couple with her. To defeat him, she turns into a bird, a tree, and a tigress. The latter scares him off, but eventually he catches her while asleep (XI.225ff.). - RBW