For others reading this post, Steve's comments are bang on - provided that the fiscal year 2011 has not yet been created. Once fiscal year 2011 is opened (as a Next Year), then 2009 is off-limits to post to. The original entry may have only been a couple of months ago, but it certainly is important to know what processing has been done since then.
If you haven't gotten that far yet, and 2010 is still your current year as defined in the company data, then you still will be able to create a batch and 'Post to Prior Year' to update 2009 (assuming that the ability to Post to Prior Year has been enabled in the GL Company Profile and security allows access).
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Regards,
Softrak Tech Support