I have been using AdSense ever since it first started and this is a feature I have always wished Google would make available. They have announced that it is coming soon.
You will be able to manage your AdSense ad units from within your AdSense account. You will no longer need to manually replace the ad code on all of your pages.
Google says they will be rolling it out in phases in the next few weeks.
Here is a link to the information from Google which contains more details:
Inside AdSense: Coming soon: Make ad changes without replacing code