does turnitin check old papers

Another technique that students seem to be using is turning in their old essays. They ask themselves ‘Can Turnitin detect my own work?’ and they will take an old paper from another course and hope it sticks. If it’s carefully rewritten, you might be able to confuse Turnitin. However, if the paper was previously submitted for anti-plagiarism scan, you can bet everything you have that Turnitin will remember that. So the next time you’re thinking ‘It’s not plagiarized if I wrote it,’ think again. Turnitin doesn’t like recycled works either.
To answer your previous question: yes, Turnitin can definitely detect copy and paste. If your paper has content copied from elsewhere that was not properly referenced, Turnitin will find it. It’s like a really smart detective that can catch anyone cheating in the act. So if you ask us “can Turnitin detect books?” then the answer is yes. Turnitin can detect published books as fast as you can say ‘plagiarism.’ If you decide that you are ‘smart’ enough to copy a text from a book and claim it as your own writing, you might feel a bit of embarrassment later on. If you do not follow any guidelines for proper citations (such as MLA, APA or Harvard) and blindly copy and paste everything, you can be sure that Turnitin will catch on.

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Reality: Papers are secure from prying eyes. No one can go into the student database.
Reality: Turnitin has many options and settings for adapting to your various institutional, departmental, and individual needs. Instructors can decide to let students view their reports, allow students to resubmit and receive revised reports — or not.

None of that has anything to do with actually writing the same assignment from scratch twice, even if you end up making many of the same points in both papers. That’s not what students do when they purposely choose the same topic for papers in different classes.
You’re the one who apparently admits to writing by changing words for the same ideas (as implied in the first quote above). I’m not creating any “double standard”; I’ve explained several times that you can’t compare students and hired writers in this respect, simply because when students self-plagiarize, it’s almost always a deliberate choice to select an essay topic they already wrote for another class. That has nothing to do with the issue of having to write the same assignment multiple times as a writer, because you suggested that professional writers and subject matter experts often write about the same topics multiple times. Self-plagiarizing students choose topics for that purpose; professional writers don’t get to choose their topics like that.

It is possible for the Similarity Index (the percentage of matching text between a submitted document and sources in the Turnitin databases) to change after you’ve received an initial Originality Report.
Turnitin looks for text similarity and doesn’t grade papers, this remains an academic judgment to check the assignment, examine the originality report to understand what a student did and then decide whether or not there is a problem.

References:

http://www.algonquincollege.com/blackboard/how-does-turnitins-originality-check-work/
http://www.turnitin.com/blog/top-15-misconceptions-about-turnitin
http://real-atlanta.com/forum/gt/years-ago-turned-paper-turnitin-use-same-2883/
http://bue.real-atlanta.com/Turnitin2_Students/ContactTurnitinAdministrators
http://www.real-atlanta.com/what-can-turnitin-detect/