If you are wanting to migrate from All-In-One SEO to SEO Premium, we would recommend using the ‘Import from other SEO plugins’ feature of the SEO Premium plugin. Please note that prior to making any significant change or migration to another format, we would also first recommend backing up all your data prior to proceeding.
To import your data from All-In-One SEO you would need to perform the following actions:
Go to SEO>Tools
Click on ‘Import and Export’
Click the ‘Import from other SEO plugins’ tab
Yoast will detect any compatible SEO plugins it can import
Click ‘Import’
We also have a guide on using the SEO Premium plugin’s import/export features at the following link: https://kb.yoast.com/kb/import-options-in-yoast-seo/ as well as a guide on how to migrate your date from the All-in-One SEO plugin to Yoast SEO Premium here: https://yoast.com/all-in-one-seo-pack-migration/.
As for the sitemap error of ‘XML declaration allowed only at the start of the document’ The error ‘XML declaration allowed only at the start of the document’ this is most often caused when another plugin or theme adds white space to the beginning of the sitemap file. You will need to view the source code of the sitemap document in order to confirm this and we have additional information on this and how to resolve this error at the following link: https://kb.yoast.com/kb/xml-sitemap-error/
In order to resolve this, we would first recommend clearing your site’s caching. If you are not sure how to clear your server caching, please speak with your host provider. Does clearing your caching from your caching plugin (W3TC, WP Supercache, etc), server (Varnish, LiteSpeed, or others), or CDN (such as Cloudflare, Amazon AWS, etc) resolve the issue?
If after clearing your site’s cache you are still experiencing the error on the sitemap, we would then recommend performing a conflict check. You can sometimes experience functionality problems or unexpected behavior from the plugin due to plugin or theme conflicts. Can you confirm you have performed a complete plugin/theme conflict check by disabling other plugins, and resetting the theme to a default such as twentyseventeen? We have additional information on how to check for a conflict here: https://kb.yoast.com/kb/how-to-check-for-plugin-conflicts/
No further response, setting to resolved.