No harm in being picky.
The change would have been promulgated in the AIP, and elsewhere, for the AIRAC cycle commencing on Thursday 2nd July 1987. Given Heathrow's operations, it would be reasonable to conclude that the necessary changes to the runway markings and taxiway signage were accomplished between the last movement on 1st July and the first on the 2nd.
There's a reference to the designation change in this thread:
Heathrow runway change from 28L to 27L, including a contribution from Heathrow Director, whom one would expect to know about such things, confirming the 2nd July date.