Looking back to the start of this thread I note that in the July Sky News story link it was Angela Rayner, the Shadow Education Secretary, who said that there were no plans to write off existing student debt, not Corbyn. He really is being accused of going back on a promise that he never made and that he hasn't denied because he didn't make it.
In the same story it says
In a key pledge Labour promised to scrap tuition fees later this year - and Mr Corbyn also vowed to "deal with" the debts of students who have already graduated.
He told NME magazine in June he wanted to look at ways to reduce, ameliorate, lengthen the repayment period or "some other means of reducing that debt burden".
To me, this doesn't look like a promise to write off the debts, it is a 'promise' (if you want to take it as such) to look at ways of ameliorating the debt. Amelioration isn't the same as 'writing off', it just means to make a bad situation better.
I don't think the passport analogy is particularly helpful. There's only one way to renew a passport; there are several ways to ameliorate a debt. Promising to 'deal with it' just means that he intended to do something about it.