I have a network of websites that all have their own domain. I would like to combine them all into one website using one domain, but how can I keep my SEO? Should I keep the domains? What about all my link building for my old domains? Any help is appreciated. :)
A 301 redirect should pass any link juice you've accrued from the old domains to your new domain.
That said, it's better to have multiple domains and attack niches (for example: if you sell office stationary online, it's better SEO to have a website exclusively for staplers and another for pencils and pens instead of lumping it all in one website).
You can go the route of having everything under one umbrella domain, but make sure your website architecture is flat.
P.S. I made some assumptions about your website (since I have no idea what your product or service is), but my advice is applicable to anyone.
