All script files including JMX, CSVs and any other associated file(s) you might have used for the test run or you configured URL test, easily download whenever you want and export and use them in JMeter.
In case, after uploading a JMX file, you do any modification in Load Scenario settings and save the test, same changes will be overridden to the upload JMX file. On download of the file, you will see the modified content.
Also, aggregate report table can be saved as a CSV file for reference.