Can I load modified Solicitation Response (SR) values into an existing Evaluation (EV) document when the EV contains the old SR information?

No, this is not possible. Attempting to do so will yield the following error:

If any Solicitation Response’s have already been pulled into an Evaluation (EV) document and those SR’s need to be updated, the EV will need to be discarded and a new EV will need to be created to pull in the new values.