Decision by McLaren in 2019 to switch engine supplier from Renault to Mercedes is the reason to be exception with the newly imposed rules by FIA to the teams to carry their 2020 cars for 2021 season.
McLaren enters in their final year using Renault engines and will make switch to Mercedes for the 2021 season. This decision was made long before current global crisis so consequently rule that team will use same cars from this season for the 2021 will not apply for McLaren. They are allowed to make some important and necessary changes to the chassis to accommodate new Mercedes V6 hybrid.
Team boss Andreas Siedl confirmed this in interview for McLaren official web site:
“This decision will not affect our choice to start driving Mercedes engines in 2021. We have been given permission to make the necessary changes to our car to allow the engine change.”