Start a new game and use the starting given money to upgrade your weapons or buy new ones. Keep playing the game until you reach the cinematic with the moral choice. Select the moral choice and during the cinematic with the explosions return to the 'Main' menu and select Campaign mode. Choose the Campaign you just played and select 'New' and you will retain ALL your previously bought weapons, upgrades and money from the previous mission. Your weapons can be upgraded again and the procedure can be repeated to fully upgrade or purchase ALL the starting weapons.