This has happened to me - purchase ticket with expiring voucher, cancel ticket, new voucher has 1 year expiry.

So far I haven't lost any funds, but I'm not sure if it's an official policy so much as an implementation detail that lands in our favor. Sort of like the old OLCI that had a 24-hour rolling checkin and SDC window. I miss being able to change a late night flight to the next morning and vice versa.