I have recently upgraded my Wordpress site to https and i have noticed that the Yoast SEO plugin generates sitemaps that have URLS defined as http. Is this incorrect for a https site and if so is there anyway to flip it over to https?
Yoast SEO uses the default site address set by your WordPress installation which is entered the first time you install WordPress. When enforcing a HTTPS on a WordPress set you must setup the current URL on both the WordPress Address and Site Address URL options found within the Dashboard in General Settings.
Then your options should look something like this:
If you have these settings already in place then its likely your hosting is caching the current file which needs purging.
You might need to check your wp-config file. Some managed wordpress hosts or people that harden wordpress for solid security reasons will set the url in wp-config. If that doesn't match up to what it should in mysql or your wordpress settings, that can cause this issue. (and make changing it in wordpress or the database feel impossible)