Changing the maximum numbers of turns

Hello, I started a game with 100 max turns by mistake and we're quite a bit into it already. I wanted to put the number of turns on max but the option is grayed out. Is there any way to up the number of turns in-game?

You can get a mod to change them for you should everyone agree, though I'm not completely certain they can change that.

The game duration can not be changed, sorry