Hello, 

I'm upgrading from a 220-G2-0650-Y1/220-G2-0650-Y3  (need to double check exact one) to a 220-G2-0850-XR 
My understanding is that the two should have compatible cables so I don't have to rewire everything but I wanted to 2x check since it would suck to brick my build.
 
Thanks in advance!

Welcome to the forums.
 
If you go to the product pages for the EVGA SuperNOVA 650 G2 and EVGA SuperNOVA 850 G2 and scroll down to the Compatibility tab where the cable sets are shown, you will see that they take the same cable set part number and are interchangable.
 
Cables for all EVGA SuperNOVA G2/G3/P2/T2 PSUs are interchangeable which makes it nice for those who have painstakingly or otherwise taken the time to perfect their cable management and just need/want to swap out a faulty PSU/put in a different one.
 
Use the new cables, keep your current cables in place and just swap out the PSU itself, whatever you wanna do.

Should note the p+ series are different and will short out your motherboard!