Change of use of the property is defined in Article 71 of the Building Law.
You should report to the local government any change in the use of flat if its effect is a change of the safety conditions listed in the Act, namely: fire safety, flood, labor, health, hygiene and sanitation, environmental protection, volume or load of the structure.
So the content of the contract do not change much if above is applicable.
More detailed information you will find here (polish source):
Change in the use of a building or its parts.
It will be important this change, which affects the changing requirements of the object, and hence - the possibility of further, amended way of its use.