Dear Freesurfers and experts

Does anyone know how to flip hemisphere surfaces (cortical thickness) for with-in group analysis?

I am analysing a cohort of unilateral chronic knee OA patients and want to within-group correlate the cortical thickness changes with the mechanical pain threshold. 

(A perfect study would be having only Rt. or Lt. knee pain and then the hypothesis would be a contra-lateral brain cortical thickness changes) but  since I have a mixed patients of Rt and Lt knee pain, I want to flip only one group to match the other and make them all as they were having same laterality, and then do a correlation analysis using QDEC!

Any help will be much appreciated

Thanks 

Hamza