It depends on the ticket you bought.
By default tickets are for a route, not a train. Even though you start the purchase process by picking a train from a schedule that does not mean that you are limited to that train. A standard “point to point” ticket is good for any train on the route you bought it for, for a whole day. So you can take an earlier or later train at your own discretion.
So in most cases you will not “book the 1:30 train to Luzern”, you will just buy a “Zurich Airport to Luzern” ticket, good for the whole day. Trains are mass transit. Taking the train from Zurich airport to Luzern is as trivial as taking the Elisabeth line from Heathrow in to London.
And with the HFC you get these tickets at a discount of 50%! The price for a normal ticket is always the same, and they do not sell out, so there is no reason usually to buy them far in advance, except maybe for convenience.
The exception are. SuperSaver ticket. These are tickets where you get an extra discount (on top of the HFC discount) provided you buy the tickets in advance, and accept limitations. This is a fairly new thing in Switzerland. Supersaver tickets are cheaper, but not flexible, and not refundable. Do not buy a supersaver ticket if arriving by air.