Thoughts don't become things; thoughts ARE things.
Shapeshifting requires the ability to transcend your attachments, in particular your ego attachments to identity and who you are. If you can get over your attachment to labeling yourself and your cherishing of your identity, you can be virtually anybody. You can slip in and out of different shells, even different animal forms or deity forms.
Take this idea that you have that you must be attached to things and apply to it the reality that you came here with nothing, and when you leave this great Earth you'll be leaving with nothing.