If the only time you can dig them is in the spring then dig them in the spring. Just replant and the iris will do just fine. The reason people wait is so the rhizomes will be larger by mid summer.

You move them when you can get them.

The owners very obviously don't want them and if they don't care if you take them, I'd be over there digging up every iris I could find.

Go get them as soon as they peek up and you can identify them.

Take them all, and if you don't like some of them when they bloom, give them to someone who will take care of them.

It just kills me to watch a pretty garden destroyed by new owners-- or in this economy when people can't sell, their gardens get destroyed by renters. It's so sad.