The meta tags are different, so they shouldn't conflict.
Meta tags in the <head> with this format
<meta name="description" content="stuff>
should be treated like normal.
Meta tags inside the body are treated as local meta-data meta tags. They are specific to their nearest context (which should be something with itemprop or whatever). I'm talking about these:
<div itemprop="reviews" itemscope itemtype="http://schema.org/AggregateRating">
<img src="four-stars.jpg" />
<meta itemprop="ratingValue" content="4" />
<meta itemprop="bestRating" content="5" />
Based on <span itemprop="ratingCount">25</span> user ratings
- If it's in the head and has an itemprop attribute, this is considered Schema-type metadata and I'm not sure why you would include this in your <head> when your <head> already has a regular meta description tag... OR, if you are including it, why is it different, if they are both describing page content accurately? I'm talking about these:
<meta itemprop="description" content="stuff">
But I am assuming you're using Schema+the microdata format. Are you also using Open Graph metadata?
It's also not clear that (regular old-fashioned) meta descriptions weigh the ranking of a page in the results-- instead, assuming it's a good and accurate description, it's a programmatically easy thing to show under the page name or title in a search result to give more contextual information to someone. It can then also highlight any search terms that ended up in the description tag. However if a search term was only in that description tag, it should not help you rank for it, since this would be possible evidence of SEO keyword stuffing or something silly. Since, as far as Google is concerned, if your page is about 'keyword' then you wouldn't have 'keyword' only in the meta description, of course. Google should be ranking your page on the basis on its viewable content by people. The metadata is merely to help give semantic information about your content where your HTML tags are insufficient (as for example there is no HTML tag set for People information).
I'm not sure what Google does with the meta itemprop descriptions in the <head> of a document: Do users ever see that in the results? We know they can see the metadata stuff in the body in the results (like reviews). So I wonder if it's worth it to use the itemprop meta description here?
will no longer have such a long history of being cached by Google? That is, the rich data snippet pages will have to wait to be indexed by Google and this will hurt my search results, no?
If you're making changes and are worried about the lag time between bot visits/indexing, you might want to put in a request to Google to go index your page the moment you're ready for it to. You can do this via their webmaster pages. I think you need an account first of course, if you don't already have one.
This way you don't have to wait for stuff to be indexed by a bot, if it's new. There shouldn't be much time between a bot visit and any change you may get in results online.