If you drag and drop using the left mouse button, Excel will ask you if you want to replace the existing data. However, if you drag and drop using the right mouse button, a menu will appear that will enable you to shift the existing cells down or to the right, as well as several other options.
You can see video lessons showing how drag and drop works in the different versions of Excel in the following lessons:
Excel 2007 - Essential Skills Lesson 3-4 Cut, copy and paste using drag and drop
Excel 2010 - Essential Skills Lesson 3-4 Cut, copy and paste using drag and drop
Excel 2013 - Essential Skills Lesson 3-4 Cut, copy and paste using drag and drop
I hope this is what you were looking for, but please feel free to reply if I can offer any further assistance.
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