Dell doesn't support downgrading controllers.   You would need to add a new 6500E the evacuate the 6510E 
Hi, We have a customer who has a multi-member group. Recently a PS6510e was added but there is a requirement to replace the 10GbE controllers for 1GbE. Don't ask!!! I  know what your thinking. Is the best way to achieve this to evacuate the PS6510e, delete is from the group, replace the SP's and initialize it again adding it to the existing group provising the is enough free pool capacity? Thanks in advance.
Swapping out Storage Processors