Not too painful IF you have the tools needed. Did you reads this?
http://pindelski.org/Photography/2013/09/10/mac-pro-2009-part-xvii/
Right, I meant that the procedure for using lidded CPUs on the 2009 wasn't too painful.
This may all be a moot point anyway. I found a fairly good source of Lightroom benchmarks, and as best as I can tell, moving to the Mac Pro with 8 cores won't gain me more than 20% or so from my current machine.