If you are tearing down your web site and rebuilding it completely from the ground up, you should expect a lot of search ranking fluctuations. If you want to control and track as much as you can on the SEO side, it is recommended you do it in stages and split the process up.
John Mueller of Google was asked about this on Reddit and John said "I'd try to split it time-wise so that you can recognize any negative effects in each part, and take action to improve them."
The thing is, if you don't split up the move then it can be really hard to understand what site changes impacted the Google ranking changes. John said "If you do everything at once, you'll never know what to fix, and even if things end up "same as before", you won't know if one part went down, and was compensated by an improvement on the other part."
"Keep things controllable & trackable," John Mueller said.
Here is John's full answer:
You'll potentially see more fluctuations from the redesign / revamp than from the domain name change. Moving things 1:1 from one domain to another is - for the most part - a non-issue. Moving between CMSs, rebuilding a site, restructuring it: changing URLs, significantly changing the design, changing the internal linking, all of those things can significantly impact a site's performance in search (and it can go up too, it's not always down -- you can do things to improve SEO after all). If you do that and move domains, you won't know why there are changes, and that's where a lot of the site move stories come from.If you need to do both, I'd try to split it time-wise so that you can recognize any negative effects in each part, and take action to improve them. If you do everything at once, you'll never know what to fix, and even if things end up "same as before", you won't know if one part went down, and was compensated by an improvement on the other part. Keep things controllable & trackable.
Sometimes you can't split things out, if you need to revamp & move for reasons above your pay-grade, try to keep things as controlled as possible (track all the details), and set expectations appropriately. You can't remove all risks, but knowing them makes it easier to make decisions, and to determine actions to take when they happen (which might be "hire more SEOs" instead of "fire all the SEOs").
Whenever I do site migrations, even to new CMS platforms, I try to keep the URLs, content, meta data, etc all the same. But sometimes you can't and if you cannot, you do want to isolate whatever you can.
Forum discussion at Reddit.
