I am new to the forum so please excuse any information that may be missing.

I recently bought a decently priced used road bike (Specialized Dolce) that has an 8 speed cassette and Shimano Tiagra rear derailleur. I thought it would be a good idea to have an extra set of wheels since the roads and trails are somewhat bumpy. I didn't want to be without working wheels if one needed to be repaired. I found what I thought was a good deal on a pair of Mavic Aksium S. They seem lighter than the stock wheels so I figured I would use the stock wheels as spares. The issue is that the Mavic has an 11 speed cassette. Can I replace it with an 8 speed cassette and then just sell the 11 speed to recoup the cost? Or did I dig too deep a hole and need to replace the hub as well?

Thanks for any help you can provide.