I don't know if anyone's interested, but I find little things like this easier to do if you use a macro. If you use Word, you can copy the following text between the dashes:
---------------------------------------------
Sub DoubleSpacePara()
'
' DoubleSpacePara Macro
'
Selection.Find.ClearFormatting
Selection.Find.Replacement.ClearFormatting
With Selection.Find
.Text = "^p"
.Replacement.Text = "^p^p"
.Forward = True
.Wrap = wdFindContinue
.Format = False
.MatchCase = False
.MatchWholeWord = False
.MatchWildcards = False
.MatchSoundsLike = False
.MatchAllWordForms = False
End With
Selection.Find.Execute Replace:=wdReplaceAll
End Sub
----------------------------------
You can open the macro editor on the Tools menu in Word (or press Alt+F

.
Click the Create button.
Paste the text in and then hit Ctrl+S to save the changes. Close the editor.
Right click the toolbar you want to add the macro button to, and then click customize.
In the Categories list, select Macros.
In the Commands list, click and drag Normal.NewMacros.DoubleSpaceParagraph to the toolbar you want the button in.
Leaving the Customize window open, right click the macro and you'll get a list of things you can do to change how it appears. Click Default Style if you just want it to be a button without a lot of text. You can change or edit the button image from here too.
Once you're done, when you click your button it will run the macro and change all ^p to ^p^p in your document. (Single hard returns to double hard returns).
Why bother?
Well, the bottom line is, a lot of manuscripts are written and formatted for publication with five space indents to indicate paragraph breaks. When you submit a document to a publishing house, do you typically use double spaces between paragraphs? I don't believe this is normal for narrative text. But when you submit it to a forum, the five space indents are removed.
Why are they removed? Good question. I would guess that the forums don't support tabs or multiple empty space characters. But whatever the case is, when you put something online, unless you're using your own cascading style sheet, the odds are good that your indents will be toast. And besides, it's a lot easier to read online with double spaces between paragraphs (although some people may argue this, usability studies indicate it's true for the statistical majority of people).