The schematic has been completely redone since v3. V2 schematic information will not open in V4. I recommend keeping the schematic in v2 then export a netlist. You can then read that netlist into the v4 layout to update. This requires you to keep the layout and schematic separate but will result in the least amount of rework.
You could also keep this design entirely in v2 and only order in v4. Make sure to back up your v2 files. Once a v2 file is saved in v4 format it is not backwards compatible. The only way to update the schematic you see in v4 is to remove the unwanted symbol and replace it with the correct symbol and reconnect the parts. Removing the symbol will also remove the layout footprint and all corresponding traces. You would need to start over with the design in v4.
The information below explains how to work with a netlist in PCB123.