It won't happen in October.
The EU just had an election and is organising it's committees and suchlike for the next 5 years; this means that there's currently no-one for us to "just go and renegotiate" with until a new team is formed in November although I get the feeling that what they mean by "renegotiate" is to talk louder and slower ("IRISH-O. BACKSTOP-O. BORDER-O. WE. GOT. MONEY. DINARO." rubs fingers).
Sam Gymiah, Tory MP, said this morning he thinks there are about 30 Tory MPs who will stop the new PM from proroguing Parliament (essentially suspending all sittings so nothing can be passed) in order to force no-deal.
If, as looks likely, Johnson gets in it's almost certain that there would be defections from the Tory party and a vote of no-confidence; the scale of defections could drag the Queen into it as, when Theresa May goes to her and offers Johnson as the new PM, HM's advisors could advise that he is unable to form a stable government and HM could decline to appoint him. This would force a General Election and Parliament would have to ask for a further extension which I wouldn't be surprised if the other EU leaders, who all have to agree, refused.
That leaves us with two choices, revoke article 50 or a second referendum.
With the support currently experienced by the Brexit Party, revocation is out of the question so it'll be a confirmatory referendum, probably with a preference list with the options of no-deal, TM's deal or revocation.
This is just my take on it, other dystopian futures are available
