Normal tickets in Switzerland are for a route, not a train. So they are flexible. SBB calls these "point to point", which in my opinion only someone with poor English skills could com up with :-)
A lot of the confusion foreigners experience is because they expect train tickets to be for a particular train, whereas the norm is that they are for a route, and it is only in some exceptions that they are for a train.
So even if you select a particular train for Ascona- ZRH you will end up with a ticket good for travel from Ascona to ZRH, valid from 0:00 on the day you bought, till 5:00 the next day. (so 29 hours in total). Within that period you can make one trip, on trains of your choice, and you are even free to use either the old route over the pass, or the new one throuigh the gotthard base tunnel.
Normal tickets are also the same price regardless wether you buy them a year or a second before departure. So you do not need to buy them far in advance, as long as you buy them before you board the first service. But they are expensive, especially on long distances.
So for example a normal ticket Ascona - ZRH, without reduction, in 2nd class is 73,60 That includes the bus from Ascona to Locarno. In Switzerland all public transit is integrated, so you normally need only one ticket per trip.
When yiou book in advance you will also be offered some special deals.
- One of those deals is the "Saver Day Pass". The website will automatically offer that for your travel day a pass is available that is cheaper than the normal ticket. The SDP allows unlimited travel on all public transit and is thus fully flexible. This is the best option for your arrival day btw, so I hope you have already bought one, as they only get more expensive, and eventually even sell out completely as you approach the travel date.
- Another option are Supersaver Tickets. These are also cheaper than normal tickets, but are for a specific train. With such a ticket you must take the train you selected.
Note that these are special deals, not always available, and only sold in limited quantities. The reason they exist is because SBB is making to much money and so the government has forced them to offer these discounted tickets.