Your purchased themes and content are all backed up to your Microsoft SwiftKey Account. Just as apps you download from Google Play are assigned to a Google account, your Microsoft SwiftKey Keyboard content works the same way.
When you download Microsoft SwiftKey Keyboard on your new phone, be sure to sign into the same Microsoft SwiftKey Account that you used to purchase your content from the (now decommissioned) SwiftKey Store. Your purchased themes should show as available to download.
Note: All Microsoft SwiftKey themes are now available free of charge.