If my yard is used to a set watering schedule, and I am wanting to switch to longer, less frequent watering, how gradually should I do that to give the roots a chance to grow down? Do I just set the zone to what it “should” be, and add additional water if it seems stressed? Or do I set the root depth level shallow, and add to it every week or two? If I do, what’s the best amount to change? In other words, if I have Warm Season Grass, which should technically have a 9.8 inch root depth, but it’s been watered like Cool Season Grass in the past (root depth 5.9 inches), should I start the Warm grass at a 6 inch root depth and increase it X amount per week? If that’s a good way of doing it, what should X be?
No set rule on this, but I did it incredibly slow over the course of a year using Flex Daily scheduling. You just kinda need to watch the grass and see if it starts to show signs of stress. When I did it 5 years ago, I used the root depth to make those adjustments. Started shallow, 2" I think (I inherited my yard from previous owner with daily short watering), and moved it down 1" every couple months and watched what the grass was doing. Now my grass is re-trained, I have set to about at 6" at the start of the season (it goes dormant in the winter) to about a max of 8" in the late summer months, and it survives the Arizona summers very well.
Did I do it the “right” way? Probably not…could I have done it at a faster pace? Probably…but it worked for me. You could do core samples of your lawn if you really wanted to geek out! 
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What warm season grass do you have? I’ve found that not all warm season grasses have that depth of roots. My centipede grass, for example, should have roots about 6" deep (not by my measurement - that’s how deep they usually grow, and want them to grow).
I think it’s Bermuda. I call it “the grass that survived 5 months of no water when the place was foreclosed before I bought it”. It’s a mix of that, some cool season grass, and a fair amount of weeds. Its not a thing of beauty, but my main goal is to have space for my dogs and the local wildlife, so I haven’t gone great lengths to make it a perfect yard.
@rraisley is right. I have Warm Season, Bermuda. It has one most aggressive root systems of any grass and can reportedly go 6’ (yes feet) down, but the few areas I have dug to do repairs, I can’t see much root structure below 8" I could possibly keep pushing it, but I’m pretty happy with the way it looks, and the application of water right now.
Might be worth taking a shovel full and seeing what the current root structure look like.
Much as tmcgahey says, if you’re now watering to 6" deep, which isn’t awful for any grass, and want to go closer to the 9.8" Rachio recommends, maybe adding an inch each month would be a good compromise to get it where you want.
Realize that whether you choose a root depth of 6" or 8" or 9.8", it won’t affect the amount of water you get per week on average. Deeper roots can simply accept more water at one time, and then wait longer until the next watering. So regardless of root depth, you don’t use more water. A lot of people get confused about that. All the root depth is doing is trying to give you enough water at once to last a while, without running off of the soil.
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