When did the domain name change?
Are you redirecting the old URLs to the new ones?
I have a similar site which had operated for 11 years before being renamed due to a change in the company name and rebranding. I migrated to XenForo last year at the same time the domain changed.
I don't recall how long it took for the sitemap to have an impact - but I also spent a lot of time making sure all old URLs were being redirected (made more complicated due to using vBSEO on the old site with a custom URL structure).
Either way, traffic still hasn't recovered back to where it was previously - I'm considering engaging some experts to try and work out why Google isn't liking the new site as much as it did the old one.