Yes. It will affect SEO performance. Always 301 redirect from old pages to new pages.

Unless they are already 301 redirected then you shouldn’t worry too much. Optimize the website as usual. But you still need to do some analytics housekeeping making sure the blog.domain.com is being tracked in google search console and google analytics.

On a side note, I’m not a fan of subdomains for blogs, to be honest. I’d like to put everything in 1 dominant domain unless special circumstances subdomains for blogs are needed.