What if the convertible note “expires” and the note owner does not want to postpone it?

depends on the note, but if there is no clause for what happens when the note “matures”, then generally the decision is based on majority interest (you should at least that clause in your convertible note).  Generally it is not in the interest of investors to “demand” you to return the money (as if you don’t have the money, it basically drives you into bankruptcy, then everyone loses.). so ideal situation is do nothing  (so unless he have (or convince) majority holders of the note (to decide that the note must be returned), he can’t really force you to do anything either.).   Alternatively, if you want to convert, you can send a proposal and get everyone to convert, but that have high legal cost if the terms for automatic conversion isn’t spelled out.  since you have to get majority of note holders to agree to the terms of conversion. (again, caveat, this is all advice from my lawyer from previous company, and I am not a lawyer. .) also, it takes time and money for investor to try to foreclose on you. so unless the investor is really vindictive, most likely nothing happens, which is what you want.