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Hi there, thanks for posting this.

Some questions, before I can help you properly:

Have you redirected all of the old URLs across to any new ones using the 301 directive?

That duplicate content in the subdomain, is it still live somewhere or have you removed this now?

Do you know which landing pages in particular have seen the biggest loss of traffic?

Usually, a site that drops rankings when it moves to another platform is having duplicate content or redirection issues in my experience.

I really would like to help, so please let me know the answers to the above when I can.

Kevin has a really good point here. You need to input markup that tells Google that the content is yours. I find that adding self-referential canonical tags can help with this. Just be careful to input them correctly.

Hi there, it's good that you're thinking about SEO at this stage. Too often, agencies have to work with what they already have once a website has been built, so you've got the right approach. I agree with the others with regards to Google Keyword Planner - it's a great piece of kit that will help a lot. What you need more than anything is clear and logical information architecture as well. If your navigation and sub navigations are logical and the flow of "link juice" follows that structure then you will have a good start. Consider planning in high quality landing pages for each high priority product as well. If everything is clearly marked in a site map before build, you have a bird's eye view on what is needed. Also, as another poster mentioned, don't neglect your metadata, tags, markup and content.

Hi there, thanks for posting this.

Some questions, before I can help you properly:

Have you redirected all of the old URLs across to any new ones using the 301 directive?

That duplicate content in the subdomain, is it still live somewhere or have you removed this now?

Do you know which landing pages in particular have seen the biggest loss of traffic?

Usually, a site that drops rankings when it moves to another platform is having duplicate content or redirection issues in my experience.

I really would like to help, so please let me know the answers to the above when I can.