Lets assume you are moving a site from www.olddomain.com to www.newdomain.com
In order to respond to a request for https://www.olddomain.com/ without causing scary warnings you need a certificate that covers https://www.olddomain.com/ . This applies regardless of whether the response you want to send is a redirect or not.
On the other hand a request for http://www.olddomain.com/ can be responded to without needing any certificates.
Users who just type your site name will likely end up making a request for http://www.olddomain.com/ (unless you are using HSTS) but if your old site previously redirected everyone to https then it is likely that bookmarks and incoming links will use the https url. If your old site used HSTS then nearly all incoming requests are likely to be on https.
Another issue is that many browsers now have autocomplete for urls, this means that if someone starts typing the old url the browser may autocomplete to the https version.
Rather than having seperate certificates for the old and new domain it may be better to have a single certificate which covers both domains. This will allow you to host both domains on the same IP address without relying on SNI. The downside of this approach is that many CAs consider multi-domain to be a premium feature and charge accordingly.