I would say for the child, going from only child to having to share mommy and daddy can be a small challenge, but I think if you just encourage the older child to help out and make sure you include them in things, they transition well. And once you have had 2 already, it's maybe less of a transition since they are used to having to share their parent's time.
It's nice to have a baby carrier for the newbie, so you can stay somewhat mobile to chase the older kid(s) if need be. Though it is synthetic, I like the My Brest Friend nursing pillow(I'm 5'8'.) It made it possible to get up out of where you were sitting while still nursing if you needed to. TV time, outside time(especially if you have a yard), and play time for the older kids. Go on walks, etc.
I mean, it's really pretty much life as usual, but you just can't be running around and doing stuff all the time, or at least, I really wouldn't recommend it.
As far as regressing, I could see if you had just potty trained the older kid, they might get jealous or get ideas from newbie that they should still wear diapers too. I've heard of that before. If you are going to switch around anything like sleeping arrangements and stuff, I'd do it a bit in advance for them to get used to the new situation before having to get used to a new sibling. It's also good to train your kid(s) to have a little patience if you haven't already done so. Don't get what they want the second they ask for it or they will expect that even when you are busy tending to baby. You can wait until baby comes to teach that, but it might be easier to start before hand. And if you haven't already, teach your kid(s) to help out even if they are young, there are little things they can help with. They usually like it when they are little.