Layover Issue

You are allowed the change you are planning. With a USA passport there will no problem clearing customs also known as passport control or border patrol or immigration.

The problem you may find is the cost of the change fee the airline will charge you if your ticket has restrictions.

Thank you! My partner was worried that we might not be able to leave the "in transit" area and actually go through customs, but I figure they can't force us to get on the plane, so as long as customs isn't going to stop us, there's no issue.

If you are flying from Milan to Amsterdam, you are not leaving the Schengen Zone so it is like a 'domestic' flight.

You should have no 'customs' (passport control) to go through when you arrive at AMS.

Your passport control will occur when you leave for the US

Are you planning on making this change in your ticket with the airline, or just skipping the Amsterdam to US leg and flying to Spain instead? If you do that, the US -bound flight from Amsterdam will be canceled and you will get no refund.

We're not planning on changing the ticket. We're forfeiting the Amsterdam-US leg of the flight and continuing our vacation in another direction by spending a few days in Amsterdam, then flying to Spain. The US-Italy trip was a package deal, and we decided we didn't want to come back to the US after that package deal ended, so we're just trying to extend.

Customs is not the same as immigration (passport control).

• Customs is the check on goods. Checking the contents of your luggage to see if it contains any forbidden items or items for which customs duties (taxes) must be paid.
• Immigration (aka passport control) is the check on people. Checking whether you are allowed into the country and if so for how long. This is where your passport is stamped if necessary.

Italy and the Netherlands are both in the Schengen Area, so no immigration (passport control) between Milan and Amsterdam. You will go through passport control for leaving the Schengen Area when you depart from Amsterdam.
You just walk off the plane at Amsterdam. But I see two problems in exiting at Amsterdam and throwing away the second part of your ticket:

1) The financial loss. Is there no way to cancel (and get a new cheaper flight to Amsterdam), or change the dates.
2) When they see your ticket at Milan, they will label your luggage through to the final destination. You would normally not collect it until you arrive in the USA. At check-in in Milan, you need to make sure your luggage is labelled to "AMS".
The check-in might object to this, and worst case they might refuse, and insist you fly both legs or neither.

Personally, I would cancel the whole ticket Milan-USA, get some money back, and buy a new ticket on a cheap airline Milan-Amsterdam (or to Spain).