Caloojouc12 Hello,
This situation is very common because you can buy tickets six months in advance and the planning of construction works is not known.
I think your train could be rerouted over St. Margrethen without change. Be patient and wait until the new timetables are published. We will help you if necessary. The communications I found are:
Due to construction work no local-trains can operate between Feldkirch Bahnhof and Buchs SG from 10/08/2024 (08:00am) through 09/09/2024 (04:00am). A rail replacement bus service is being arranged.
Due to construction work, some long-distance trains will be rerouted via St. Margrethen between Buchs SG and Sargans from 08/10/2024 to 09/09/2024.
If your train is not rerouted, but other trains later, you could perhaps exchange your tickets for another train. What kind of ticket do you have? Supersaver or full price?