nothing has been cast in stone

Dear teacher:

"The stakeholders have the greatest ability to influence project outcome at this stage because nothing has been cast in stone yet."

Could you please tell me the meaning of "nothing has been cast in stone"
Thank you very much.

[Tdol]

When something is 'cast in stone' it is fixed and cannot be altered, so it means changes can still be made.

[MW]

The Ten Commandments were carved in stone and could be broken but not altered.

Once wet concrete has dried it can not be changed, only broken and removed.

Until the stone is actually carved, the design can still be changed. But once carved, it can not be changed, only destroyed and then a new stone must be carved.

Same with written and fully executed contracts.