ZydoSEO - 3:11 am on Aug 1, 2012 (gmt 0)
One might ask the same question of content. What if you change/update the content on a page two months after its first spidered? Does that content become less valuable once the engines notice it has changed? I doubt very seriously any such devaluing occurs.
And links (whether navigational or contextual) are part of the content of a page. And I have seen zero evidence that they get devalued somehow because they were not there with the page was initially crawled.
If you think about what you're asking, adding links to content that has already been crawled is absolutely natural... in fact... it's an absolute necessity.
Think about how you build a new site. As you add new pages you HAVE to go back and link older pages to them somehow in order for the new pages to get crawled (unless you plan to have a site with a bunch of orphaned, non-interconnected pages using only a sitemap.xml to help the engines discover those pages... but then the "web" would not be a "web". I wouldn't recommend it! LOL)
Additionally, very few pages on the web remain indefinitely as originally published. In general pages tend to change over time... Even articles/posts which might remain the same, the templates around them (sidebar widget contents, navigation, etc.) change.
If such devaluing existed, I would think pretty much every site on the web would be guilty of doing this. Very few sites (or pages for that matter) get published and never change. Wikipedia an extreme case of where this very thing happens all of the time on massive scale. They don't seem to be hurting from it.