Question : File System Task to rename file giving failed to lock variable error

I have a File System Task in place to rename a file
File Exist in the following folder in this format [This is the Source]
This file would be available daily,with current timestamp
C:\A_20081029.txt
File at source to be renamed to [At Destination,which is the same location as Source]
C:\A.txt

Here is what has been done so far in File System Task
Is Destination Path Var : True
DestinationVar : User :: DestinationFileName
OverwriteDestination : True

Is Source Path Var : True
Source Var :: SourceFileName

In Expression Tab
Create new Expression
Source : @[User::SourceFileName]+ (DT_STR,4,1253)YEAR(GETDATE()) + RIGHT("0" + (DT_STR,2,1253)MONTH(GETDATE()),2) + RIGHT("0" + (DT_STR,2,1253)DAY(GETDATE()),2) +".txt"
Evaluate Return : C:\A_20081029.txt

SSIS Tab > Variable Created as

SourceFileName  /  String  /  Value : C:\A_
DestinationFileName / String / Value : C:\A.txt

When I execute,I get an error mssg as below

TITLE: Package Validation Error
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Package Validation Error

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ADDITIONAL INFORMATION:

Error at File System Task [File System Task]: An error occurred with the following error message: "Failed to lock variable "C:\A_20081029.txt" for read access with error 0xC0010001 "The variable cannot be found. This occurs when an attempt is made to retrieve a variable from the Variables collection on a container during execution of the package, and the variable is not there. The variable name may have changed or the variable is not being created.".
".

Error at File System Task: There were errors during task validation.

 (Microsoft.DataTransformationServices.VsIntegration)

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BUTTONS:

OK
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Answer : File System Task to rename file giving failed to lock variable error

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