There could be many reasons for this. It all depends on the changes you are making to the pages!
If you make changes that improve the pages content there would be no real reason for you to drop.
If you had a page ranking for "SEO" and then you changed the content so the keyword "SEO" was no longer included but "website optimisation" was.. are you saying it would take longer for you to rank for "website optimisation" then if you just set up a new page? If this is the case, then it makes sense! If the keywords you want to be found for change, I would create a new page and if you are worried about duplicate content - then 301 the old page to the new one.
However, if you still want to be found for "SEO" and you are simply making changes to the content, as long as you add good informative content that still includes your keyword you shouldn't see any major drop.