Should same-type conversions with a different dshape cast their input to conform to the specified dshape?

[cpcloud]

Sometimes we may want to change the types of some of the columns in a table or cast an int64 array to a float array. Should odo do this?

For example:

import pandas as pd
df = pd.DataFrame(dict(ints=list(range(5))))
newdf = odo(df, pd.DataFrame, dshape='var * {ints: float64}')

We already do this when going from type a to type b, but not when going from type a to type a.

cc @llllllllll

[llllllllll]

I personally feel like this would be a cool and useful feature.