If the contract you signed before flying is different from the one presented at the gangway, in port, or onboard — refuse to sign the new one. 'Contract substitution' is a known abuse pattern. Under MLC Standard A2.1 you have the right to a SEA you can read and to a copy of it. Substituting a less-favourable contract after arrival is the textbook indicator of a forced-labour or trafficking pattern under the ILO Forced Labour Convention.
MLC 2006 Standard A2.1 (Seafarers' employment agreements — terms, copy to the seafarer, advance opportunity to examine and seek advice). ILO Convention 29 (Forced Labour) + Palermo Protocol — passport / document confiscation + contract substitution are recognised trafficking indicators. ILO Convention 181 (Private Employment Agencies).
Disclaimer. General information only — not legal advice. If the substitution is structural (different ship, different principal, materially lower wages), treat as a possible trafficking / forced-labour indicator and contact ISWAN before signing.