One way is to use _convert to coerce the symbolic coefficients to the Real Field:
sage: var("x y")

There's a ticket -- trac #12577 -- which proposes to expose this (by which I mean get rid of the initial _). Unfortunately the convert method without an underscore already exists, and doesn't seem to be really useful, but we could use something more explicit like change_coefficient_ring. Actually, it could even take a function argument as well as a ring, and could be used for quick transformations without needing to implement an expression walker..