There is an intriguing "perspective" in the current issue of science magazine (p.1725) about proteins that can adopt different folded conformations under different conditions.
The most interesting new example is Lymphotactin, which can form a monomer with alpha/beta structure, or a dimer with no alpha helical content at all. This is published in PNAS, 105, 5057, 2008.
Way back when I have also written about proteins switching between alpha and beta structure, mainly in the context of amyloid formation by alpha helical proteins:
Groß M (2000): Current Protein and Peptide Science 1, 339-347
Proteins that convert from alpha helix to beta sheet: Implications for folding and disease
But as I never got round to learning Greek, I didn't think of calling them "metamorphic proteins" as the Science piece now does. Damn. Another missed opportunity ...