OEM specs for the 18" wheel on the 2013 CX-9 GT are as follows:
Tire: 245/60/18
Rim: 18x7.5 +45 ET
PCD: 5x114.3
OEM specs for the 18" wheels that come on lower trims of the CX-9:
Tire: 255/60/18
Rim: 18x8 +45 ET
PCD: 5x114.3
Both CX-9s have a center bore size of 67.1mm. 2013 OEM wheels/tires have an overall diameter of 751.2mm. 2020 wheels/tires have an overall diameter of 763.2mm. That difference will result in a 1.57% difference in speedometer readings between the two, but the difference is minimal (30mph with the 2020 wheels would actually be 30.5mph with the 2013 wheels, for example).
The 2013 wheels will fit just fine on the 2020. If you haven't purchased tires yet for the 18" wheels, you could do 255/60/18 and there would be no...