If Statement help / Checkbox override

I currently have the following If Statment in my field called [terminal - internal]

long story short there comes a time where I need to override this information with the info from another field called [alternative terminal]

I've created a checkbox and I wanted to see if I can modify this if statement so that when the checkbox is checked the value in [terminal-internal] becomes the value that in [alternative terminal]

I hope this makes sense - pretty much override the the [terminal-internal] with the [alternative terminal]

If(
[Port of Loading]="Vancouver", [Voyage Dates - Terminal2],
[Port of Loading]="Seattle", [Voyage Dates - Terminal 3],
[Port of Loading]="Tacoma", [Voyage Dates - Terminal 3],
)