We do a fair number of projects for new websites under new domain names but most projects we do now are redesigns or upgrades of existing websites. With new websites, you don't have to worry about what page URLs (addresses) used to be and if they're changing but with existing websites, it's a good amount of work or it may even be quite a bit of work.
The first thing we try to do is keep the same URLs that were being used on the old website. This way, any incoming links or bookmarks work perfectly. All the "link juice" (probably a topic for another article - just know that some incoming links are more powerful than others) is kept in tact - versus when a URL needs to be redirected to a new URL, some of that link juice is lost (about 15% in most cases).
The usual reason a URL might need to change is if you're changing the software that runs your website. For example, if you're going from a VirtueMart (Joomla) website to Magento, then there will be a lot of URLs changing. This is because the structure is different and they have to change.
As we do this work of setting up redirects, we need to make sure we get all the possible URLs for your website. This takes some research. We need to look at your Google Search Console (formerly Google Webmaster Tools) account and use other tools and reports.
At that point, we'll have a good list and we'll have to figure out which URLs need to be redirected.
No, they don't all get redirected. This is because some URLs might be junk. They might be ones that are completely worthless or ones that spammers or hackers have tried to create.
So, the bigger the list we come up with, the more work we have to do to sort through the entire list and make decisions and what needs to be done with groups of URLs.
They are called "301" redirects because one that has "301" specified means it's a permanent redirect (versus a "302" which means temporary - or you might recognize that "404" means "page not found" and so on). These redirect commands are typically set up in what is called a ".htaccess" file that typically sits in the root directory/folder of your website.
What we will do is sort through the list we have and set up redirects from old pages to new pages, if needed. We'll match them up. We might need your input on some of these URLs. With the ones that remain, we'll determine which of those URLs have any significant link juice and then redirect those to either a relevant page or your site's home page.
This is one of those kind of hidden things that most business owners don't know about but it's something that really matters. If it's not done right, your website could lose some really good rankings and then fall in rankings. If you're starting a website design project with a web design firm (besides us) and they're not talking about this, then you better make sure your project includes time for this work because it needs to be done. We include this with all projects.
There are some things that will greatly increase the amount of URLs and time needed to do this work. For example, if you have a Joomla website, there are a lot of obscure URLs that Joomla creates that need to be dealt with. Also, if you website uses any kind of translation into other languages, there could also be a lot of work to do. These are the only conditions that would cause more time to be spent doing this work but it's two examples.
If you have any questions on this, please let your Project Manager know.