TLDR;
- applying for a Green Card (I-485) means you cannot re-enter the USA on your TN status nor get a new TN status.
- theoretically, you can apply for a work permit aka EAD (I-765) and get it under 90 days... but the reality is that it could be 9-12 months or longer if you're unlucky
- If your TN expires before your I-485 is approved, you can stay in the USA but you can no longer work... unless you have the I-765. This is why you want to have as much time left on your TN as possible, to avoid any gaps.
- You also cannot apply too EARLY for an I-485. If you apply within 90 days of your last entry to the USA, there's a presumption you entered with immigrant intent and that's no bueno.
Also don't forget the most important part: You must be ELIGIBLE to get a green card!! for TN holders this is usually family sponsorship (I-130) or possibly employer sponsorship (I-140).
If you don't yet meet I-485 criteria, i'll pass on the advice my lawyer gave me: "Fall in love with an american and marry them."