12.6.3 Tyre specifications will be determined by the FIA no later than 1 September of the previous season. Once
determined in this way, the specification of the tyres will not be changed during the Championship season
without the agreement of all competing teams.
I haven't found anything else related though.
on a separate note: this was changed from 2009 to 2010
2009 regs wrote:12.7 Tyre Gases :
Tyres may only be inflated with air, nitrogen or CO2.
2010 regs wrote:12.7 Tyre Gases :
Tyres may only be inflated with air or nitrogen
offtopic but I couldn't resist breaking into laughter when I opened the 2010 regs. the title of the document states "Stable Technical Regulations"