It's tempting to combine the domain change with a spiffy new version of the site, but in my experience you'll retain maximum link juice and ranking power if Google's automated processes can readily see that nothing changed but the domain.
Yes, and it is one of the few things in the land of Google that actually makes sense.
I've done it both ways. Changed the domain name and then updated the site and updated the site and changed the domain name. I recommend the latter over the former, but for gosh sake, do NOT do change the domain and change the content at the same time. I mean, come on folks--that's a brand new website to any search engine. But, yeah, you see otherwise well run firms try to do it.
And any change is a change. WidgetXYXinc.con to WidgetXYX.con is a change.
Yes, I know everyone here knows that. But not everyone here knows that everyone does not knows that. And if the client comes in and says new site, new domain and we'll pay you $$$, need it in a month and you just take the money without looking at potential search results impact and advising them, you're not a professional.