Hello Xiechoaquoa84
What kind of rail pass is it? It is preferable to know the correct pass to give correct advice.
A change means a cancellation and repurchase. There is no amendment to already existing passes.
A cancellation can be done on-site, provided the pass was purchased from SBB (and not any other agency) and it is not or has not been active until that point. There is no retrospective change of already valid passes. Thus, if your rail pass becomes valid before you arrive in Switzerland, you have to call the SBB Contact Center: +41 51 225 78 56 (24/7, waiting times may occur).
Kind regards,
Roland