I have seen them change instantly...and I have seen it take a week.
If you are using Google Webmaster Tools, make sure the sitemaps are updated.
I have found that helps me.
Also pinging the site sometimes helps.

Is this a matter of just waiting for the spiders to crawl or am I missing something?

Sometimes it depends upon visits by adsense bot. Maybe your page haven't received a hit yet from the mediapartners (adsense crawler) bot.

A single click rectified the issue(no not my own), which explains exactly why the ads needed to be changed ! thanks for the info