I suppose I may be confused, but I prefer to write this type of query with isnull...

where isnull(@Gender,Gender) = Gender

and isnull(@age+1,age)<=age

gives the same results as the nested...

Here's what I would do:

1) detach your datafile (right-click database name -> tasks -> detach). This makes the .mdf file available for copy/rename.

2) copy & rename your .mdf file...