can SPDT relays replace SPST switch? Or does it depend on the particular implementation of the relay?
If all the ratings are the same, then yes you can use the SPDT in place of the SPST, you will just have an additional terminal on the package that is left floating that does not exist on the SPST. If there is already a PCB design utilizing the SPST, the PCB design may need to be updated for the new relay.

edit: Just to clarify, you used both the terms "switch" and "relay". My answer assumes you are talking about a relay.