I'm not using the raising/lowering indices operations to switch from rectangular to polar coordinates, I'm using them to switch, in polar coordinates, from a contravariant basis to a covariant basis.
Actually, I think I know what I got wrong. The correct formula is...
Using tensors, I'm supposed to find the usual formula for the gradient in the covariant basis and in polar coordinates. The formula is \vec{grad}=[\frac{\partial}{\partial r}]\vec{e_{r}}+\frac{1}{r}[\frac{\partial}{\partial \vartheta}]\vec{e_{\vartheta}} where \vec{e_{r}} and...