I've got it on my site as the summer of 1966 but I'll have to check my sources for a more precise date (may not be possible as it obviously wasn't a one day change).
Photos of the BUA VC10s are here:
G-ARTA:
Histories - G-ARTA (never wore the first scheme as it was bought in 1969)
G-ASIW:
C/n 819 - G-ASIW - 7Q-YKH
G-ASIX:
Histories - G-ASIX
And here:
Other Operators
Edit: I'm not that sure that the tail bullets were all white
gruntie, if you look at the second link for G-ASIW you'll see that this first BUA VC10 already carried the dark blue tail bullet when under test at Wisley, before delivery to BUA.