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There's nothing wrong with upgrading your site.  In general, I would recommend to update your site, if you have the time to do it right.  Things to watch out for are: will every page still have a unique URL?  There are a number of HTML5 based systems which run the entire site on a single URL (look at how Gmail runs for example).  It can be great for an end user experience since the pages appear to load very quickly.  However, such a system would probably make it difficult for all of your pages to be properly indexed. assuming you'll have unique URLs - make sure that the new site will use the same URLs as the original site.  If not, plan to put in 301 redirects for all of the existing pages. just because you're going HTML5, don't forget the basics.  Use your title tag, H1, H2, etc on the new template. after you launch the new site, make sure spiders can still crawl the site properly.  If yes, then you should be all set with your new site.

Thanks for your time Brent. As far as I know I dont have any configuration that could block spiders. I have more sites in same sever without any issues, nothing special in .htaccess and some blocked directories in robots, thats it. Actually google is indexing the web site very well (about 50% of urls submited two days ago). I am sorry if next question sounds like blasphemy here in the comunity: do these tools really works? I dont want to expend time solving false errors. Also my free month is about to expire and would like to know it worth to pay next month.