So the trick is to do this

Lock in your new connection date. Do not talk to your old ISP yet. Also take care if your old ISP has a minimum notice clause in the contract, if it does then find out the minimum notice you need to give and chose a date around that, so you're not paying for a connection.

Once Quic have confirmed the date and time, and you get an email about Chorus (or whoever your local fibre company is), then call your old ISP and say I want my account to close on (insert 2-3 days after your Quic account becomes active). Doing it this way means you might pay for a few extra days on your old ISP but it's worth it in case something goes wrong.