The decision to switch beds should not be made during the niddah period. Even if a couple decides to switch beds during the niddah period, the husband may not sleep on the bed that was formerly used by the wife. (There is a leniency allowing the wife to sleep on the bed formerly used by the husband.)
Since you share the same bed while you are tehorah, you should make a conscious decision while you are tehorah that your husband will continue to use that bed once you become niddah, and you will move to the couch.
Please feel free to get back to us with any further questions.
B'hatzlacha!