No. As far as I know from my experience it doesn't cause harm for Responsive.By any chance it does, it is temporary and will be ok soon.Basically result never get down if you do not do any major changes like converting your site design into a new one.

You can pretty much bet it will impact their SEO because you said new template.

Existing internal links, URLs, page titles & <h> tags are the most important on-page items.

Since you're taking the same content (and same URL) but making the website responsive, drops in SEO, if any, will only be minimal.

It all depends on what exactly you are doing - are they on something lIke Wordpress at the moment? and keeping it with Wordpress and just changing the theme?

If URL's are staying Exactly the same, there is no issue here. If you are keeping the desktop sight exactly the same, there will be no issue here, if you are just changing the CSS to make their current design responsive, there should be no issue here either.

But if its a whole new design, new layout, new headings in different places, then yes this can affect SEO and Google ranks. But without knowing exactly what you are doing, or what they are currently on, its impossible to say what will happen.

It can yes, be careful which languages you use and structure.

If you keep the url structure the same, there should be little or no drop in search engine rankings.

There may be a slight change if the code to text content changes dramatically.

I'd expect the responsive design to help with mobile queries when the mobile algorithm updae happens on April 21st

Thanks everyone for your advice.
I will clarify the project a little more for whoever wishes to reply.
The client website is a static website - fixed width and looking rather old. He wants to convert to responsive and i suggest he should choose a design from themeforest that is responsive and latest HTML5 that looks as close to his own layout - if that's what he wants. But i suggest that regardless of the template layout, i can rebrand it with his logo and colours and replicate all the pages and links as in the old site. I explain that the newer premium templates have superior coding and proper use of tags like the meta viewport tag which Google recommends for being indexed as a mobile optimised site and adding more serp points to boot. All content - headers, paragraphs, pages and outbound links will be the same on same server and no wordpress either. I might suggest a simple cms, as yet to be decided but possibly a simple one like couch cms.
My own feeling is that it will impact on ranking for maybe a few weeks and then jump back up and hopefully go further up as a responsive site.

I think so that you might be aware of the algorithm update that any websites with responsive design are more prior to rank better than websites without responsive designs. So getting any websites with responsive designs have a better impact of ranking better.

It won't affect your SEO if you can save the structure of your website! But, it's worth as your visitors won't have problems if they are using DIFFERENT resolutions to open your website!

No, if you are not removing website meta tags, robots & sitemap files. It will work same.

Thanks guys. I personally think it's well worth updating an old website to responsive with the added benefits of Google indexing mobile optimized sites as of 21st April. But clients can be a stubborn lot and it's difficult trying to prove to them that a slight serps setback will be rewarded in 1 - 2 months with a leap forward.