Option 1
Make 2 separate reservations: 1 week convertible + 1 week station wagon.
Enjoy the savings of weekly rate + suffer 3 days in s/w instead of conv.

Option 2
Make 1 reservation: 2 weeks convertible.
Contact location's manager(s) for approval of planned switch on day 10.

If they refuse - offer to make reservation directly with location. If they can save on referral/commission fees, they'll be more flexible with your special request.

If they agree - make sure to get it in writing. In appreciation for their assistance, @:-) ask if they'd prefer for you to make the res. with them directly.

Food for thought
Do you care about earning miles or points with this rental?
With 2 separate rental contracts you should be able to gain more points. Ex.:
1 contract = 1 pile of bonus points.
2 contracts = 1 pile with 1 program, 2nd pile with different program.