Website migration SEO is the process of preserving (and ideally improving) your search rankings and organic traffic through any major site change: a CMS or platform switch, a full redesign, a URL structure change, or a domain move.
The risk isn't the change itself — it's what commonly gets lost in the process: missing redirects, changed URLs with no mapping to the old ones, dropped meta tags and content, or a new site structure search engines have to essentially rediscover from scratch. A migration plan exists specifically to prevent that.