The registration doesn't expire and they don't even charge you a fee to maintain your status. The PTO has added to the undergraduate degrees that they will accept as qualifying. Check out on the General Requirements Bulletin . If you don't have a degree the PTO will accept you if you have enough credits in hard science or engineering. If you are close there are community colleges everywhere at which you can pick up what you may need.
Flexibility is the key to success in the legal profession in the future.
Grab every qualification you can. The PTO registration number is the prize, which one way or the other will be of great advantage to you in the future. A detailed list of dates and locations is available.