How to draft a contract between two parties: A step-by-step checklist Know your parties. Agree on the terms. Set clear boundaries. Spell out the consequences. Specify how you will resolve disputes. Cover confidentiality. Check the legality of the contract. Open it up to negotiation.
No. It is not possible for one party to unilaterally change the terms of a contract, since contract law requires that both (or all) parties agree to the terms provided and signed.
Legality: A contract is a legal document and a PO is a commercial document. To elaborate, a PO is an offer from the buyer to the supplier. If accepted by the supplier (within a reasonable timeframe) the PO becomes a contract. Logically, this PO will have zero value if it's not accepted by the supplier.