Social Insurance Number

I am currently living in the US and have been for 16 years. I became a US citizen. I am preparing to return back to Toronto (permanently) to care for my sick mother and so have renewed my Canadian Passport (hoping there will be no problems). My question is regarding my old SIN. Will I be able to re-establish this (I still have old SIN card) or do I need to re-apply for a new number?
Can anyone help with this question? Thanks. C.

Your SIN is yours for life, same as your SSN. You may need to notify HRSDC but I dunno. I'm sure the info is on their website.

Your SSN or SIN is yours for life, the only exception is if you enter Canada in a non-immigrant category and get an SIN that begins with "9", if you then become an LPR they will issue you a new SIN. If you're a Canadian citizen your SIN remains the same forever.

If you have an SSN in the US, even though it may no longer be valid for employment, it's still valid for everything else. If you later enter and become an LPR or enter in again in a work authorized non-immigrant category, the SSA will simply mark it in their system as being valid for employment again.

Bear in mind the tax issues of leaving the US, which are covered in various threads in the US section. You must still file a 1040 every year in the US and file a 2555 (or 2555-EZ) with it. Read the instructions for 2555-EZ carefully because it's actually pretty straightforward to do, people seem to get confused about it though. Basically you just bang on the 2555-EZ what you earned during the year and deduct it from your reported income on the 1040, so it usually works out to zero unless you earn over $87,600.