Is a person entitled to claim half of their partner's share of the pre-agreed profits, after having exerted additional effort based on a promise from the partner, and are partners entitled to change the original agreement after one year of work?

The partners are not entitled to withhold the agreed-upon 30% of the profits from its owner. They must give him the full percentage, even if it is more than what the other partners receive, because the profit of each partnership is determined by the agreement. As for the shareholder's promise to give you half of his share in exchange for your help with a task outside the scope of your work at the company, he must fulfill it, because it is considered a form of ijarah (definition) (hiring/leasing). If you both agreed and you performed the work, then the compensation becomes binding, either as a percentage of the profit or the customary wage (ajrat al-mithl) if it is not permissible to make the compensation a percentage.