Can I upgrade the SSD on late 2013 retina Mac Book Pro? Do the late 2013 models (13" - (ME864LL/A, ME865LL/A, ME866LL/A), 15" - (ME293LL/A, ME294LL/A)) support after market SSD upgrades? If so, is a drive that works with the late 2012/ early 2013 rMBPs compatible with the late 2013 models?

Although not officially supported - A couple sources (EveryMac, OWC) indicate the late 2012/ early 2013 rMBPs support after market SSD upgrades. 

[Update]
ifixit posted a teardown here indicating in step 8 that:


  The proprietary SSD has changed to a PCIe format, but still isn't a standard
  2.5" drive. However, it is a separate daughtercard, and we’re hopeful we can
  offer an upgrade in the near future.
The current OWC PCIe SSD's for Macbook Pro Retina's 10,1/2 work for the 2012/early2013 models. Not the Late 2013 models (11,1 (13-inch models) &amp; 11,2 (2.0 GHz 15-inch model) &amp; 11,3 (2.3 GHz 15-inch model) ID#'s).

They claim they will release compatible upgrades "for next year". So, I guess, not until 2014. Could be sooner, I don't know the history of their estimates->outcomes of SSD releases for pre-announced/expected products.