Swap CPU in late 2013 Mac Pro?

a) that particular swap would be a downgrade, the XEON is superior.

b) No. they are not interchangeable. The firmware expects a certain series of XEON processors. Nothing else will run.

Find the same family of Xeon model and yes they can, just not with desktop i7 type.

Intel Xeon E5 with 12MB L3 cache and Turbo Boost up to 3.9GHz

Configurable to 3.0GHz 8-core processor with 25MB L3 cache or 2.7GHz 12-core processor with 30MB L3 cache

http://macperformanceguide.com/blog/2014/20140306_1-2013MacPro-upgraded.html

http://eshop.macsales.com/shop/turnkey/MacPro2013/2013_Xeon_Processor/Apple_Mac_ Pro_2013

http://www.everymac.com/systems/apple/mac_pro/specs/mac-pro-quad-core-3.7-xeon-e 5-gray-black-cylinder-late-2013-specs.ht…