Air France and KLM are relocating back to Terminal 4 on Sunday 26 March 2023. A move was expected beforehand but it seems likely that it will occur at the start of the IATA summer season which commences on Sunday 26 March.
This move by Air France and KLM was requested by Heathrow Airport Limited due to expected congestion at Terminal 3 this summer as LHR returns to pre-pandemic passenger numbers.