861s Prior to 861-100450 cost more to upgrade than one with a SN > 100450. I think when I investiagated some units on agone about 3 months ago the price difference was about $1000 for units undr 450 which needed to go back to England.
back has $3955.00. I beleive there was about a 10% increase in Junne bringing the price up to about $4300. I'm still waiting for a quote on upgrading the units I own now: an 800 v3 to v4 & and 861 v2.81 to v4 (sn >100450).
I have asked dealers about upgrades and a couple have said that they don't have much room to deal saying that the margins on the uprgade were low.

861v4, 800D, DSP5500 L/R G57, B&W rears,
Fanfare FM FTA-100, SIRIUS SAT RADIO, AppleTV, Comcast X1 DVR, Revox B215, Sony 60XBR800.