I do this and it works good for me. I don't have to dig them just pop out the old rhizomes or the ones that are to thick. I try and stay on top of this every year so I still get blooms every year from the undisturbed iris. I usually use the fork used for weeding weeds like dandilions, but get a good one the cheap ones bend easy and I keep it sharp. With the fork on the end I can find the spot where the rhizomes join and just push it down through cutting them apart then pop out the ones I want gone. This pic is a flimsy one but I couldn't find a pic of the stainless one I use.

Great info! Thanks

Extremely interesting and valuable suggestion, Kent.

I've see very large iris beds, in the past, and wondered about the logistics of their maintenance.
Unfortunately, I have seen several in public gardens being replaced.

I've also used iris in several large mixed perennial gardens, where there was room and your suggestion is well worth me following.
Example below:

I do not understand. How do you know the old from the new and what if you pop the wrong one out. Do you have a close up maybe? Where does the knife go, I mean where do you slice it at?