Yes - by then the only "front line" users of Centurions were in Hong Kong I belive ( A single squadron ?).
I cannot think of any unit that would have gone from Cents straight to Challengers.
As an example (History of 2RTR):
http://www.army.mod.uk/2rtr/regimental_history/
So - 1968, Centurions out and Chieftains in for 2 RTR.
Re-roled 1972 as a Recce regiment, back to the heavy role in 1984 again with Chieftain, and then CR1 in 1986, CR2 in 1998.
After the early 70's its highly unlikely you would see any Centurions other than the odd squadron in say HK. Most were probably sold off to other NATO countries by 75 or so.
UK regiments would block-convert to a new type post war (unlike WW2 when you got what you got given, unless pulled to the rear for a full refit. Saves on the logistical burden. Possibly, there may be a squadron by squadron roll in, but never troops in the same squadron mixed, other than adding conqueror troops to centurion squadrons briefly at one period.
Cheers
Andy