You are quite correct, Excel do have an agreement with Miami Air to swap aircraft in the summer / winter seasons.
But this summer excel are not taking a unit as Miami Air need all there aicraft. In past years excel have used the reg: G-OXLA for the leased unit.
For this year they Excel have taken a SAA B737-800 with winglets. The reg is staying as ZS-SJP and is in Excel livery.
Hope this helps .....