I think my wife's dislike it motivation enough.
I understand. $20 + gas and a little time to make her happy seems to be well worth it.
From the sounds of it, she would not like conspicuous rears (as in using the old speakers as rears even if only moved out temporarily) unless you have a good storage space for them when not in use.
I keep hoping someone with 7.1 experience in a bigger room will post. Given your room,
from what I have read you will benefit more from 7.1 than those of us with small rooms.
Spend the $20 and bask in the glow of adoration from your wife!
That will solve one problem. Then, use the old speakers for a little testing to see if 7.1 makes a difference. If it does, upgrade the rears to something nice.