But
In the F1 Sporting Regulations it says (rule 76)
So they are allowed to change the rules halfway tru the season based on whatever ground they deem fit.- if, in the interests of maintaining current levels of circuit safety, the FIA deems it necessary to reduce
tyre grip, it shall introduce such rules as the tyre suppliers may advise or, in the absence of advice which
achieves the FIA's objectives, specify the maximum permissible contact areas for front and rear tyres.