Just got off a call with a KF agent, and here's what I found out.
1. Date change. If the change only involves a change of date, there will be no further charges/recalculations for as long as the ticket is valid (1 year from issuance). If there is a date change after 1 year from issuance date, it has to be re-issued. In this case, the ticket will follow the new TNCs, any YQ paid will be refunded, and you are required to top up any miles difference.
2. Flight change. If there is a change in flight, but to the same destination and in the same cabin, point 1 applies. If there is a change in destination or cabin class, and you request it after 23Mar, new TNCs apply.
3. Upgrade. For upgrades on a paid ticket, any upgrades after 23Mar will not have the 15% discount, and nothing is refunded. For 'upgrades' on a redemption ticket, you are required to top up the miles difference, and any YQ paid will be refunded to you.
So nope, you cannot pre-book travel more than 1 year ahead and hope to change the date after the desired dates are open for booking. You can however speculate travel plans within 1 year, issue the ticket and make free date changes using old TNCs anytime.