How to redirect users from my previous support website to Guide without affecting SEO?
When moving the content from the previous help site to Guide, amend all the URL redirect directives against the hosting platform that is currently utilized for the website.
This ensures that any of the old links that might be bookmarked, or linked in external websites are properly redirected to the new ones.
The redirect directives need to be amended on your current hosting side. Depending on the server type used for your previous help website, you could amend the .htaccess file and set a 301 redirect directives rule to the new Guide website.
Alternatively, apply rules at a URL individual level or for any requests to be redirected to the new pages. However, note that to apply redirect rules at an individual level, create the articles first against the Help Center.
You can also follow the checkpoints from this guide, that explain all the steps needed to be taken when moving to a new website.
Once the redirect process has been implemented, use the Google Webmaster's tool Fetch as Google feature to help the new pages get crawled and indexed faster:
This ensures that the new pages will be prompted in Google's SERPs faster.
For more information, see the article: About search engine optimization (SEO) in Help Center.