A site I am working on moved a subdomain to another subdomain via a 301 redirect. However when checking robots.txt of the old subdomain, it has a robots.txt which disallows search engine web crawlers to crawl it. Is this the right move? I believe no because the crawlers won't be able to crawl the old site to see the 301 status codes. 
When you move a site via a 301 redirect should you setup a robots.txt disallowing robots to crawl the old address?
seo;redirects;robots.txt