Hello GrahamPudney
Welcome to the SBB Community.
Kindly note, your question is about the Swiss Travel Pass, not the Swiss Pass. The latter is not the same.
You can change the dates, but only if you cancel the passes again and repurchase. If you purchased the passes with SBB, please call SBB: +41 51 225 78 56 (24/7, waiting times may be possible). The cancellation fee, when purchased as a guest, is CHF 10 per cancellation request.
We therefore always do suggest purchasing train tickets as part of your last step in the entire trip planing, to exactly avoid this situation.
Kind regards,
Roland