I typically fly United or American internationally. I cannot figure out how to book a round trip, say to Europe that is multi-class. I'd like to book economy plus on my short domestic leg, then first or business first on my long flight to Europe, then economy plus on my third leg in Europe (also short). It just isn't worth it to pay for business first for a 45 min domestic flight that is just to connect to either DFW or IAH.

How do I do this? Or can I do this?

Is it as simple as buying the trip in economy, then just selecting business class seats for that leg?

You can do this with a multi city booking, but you will end up paying more. There may be a way using the ITA Matrix or by calling.

https://onemileatatime.com/mixed-cabin-itineraries/

You should be able to upgrade seats on individual legs of the trip after you book it. I've done this before.

Thanks!

When we're booking business travel it's often the same price or less to book everything in business/first vs booking part economy and part biz. Like thousands less in some cases.

I am going to have to watch that. And it may be airline specific. I messed around with two domestic itineraries that I currently have that are both standard economy: one on American and one on United.

In the seat selector page, the United one will let me change seats on a flight to business for the price of the upgrade. The American itinerary will not allow it.