Conversion from Swiss Pass to Swiss Flexi Pass

I bought a four day Swiss Pass (2 for 1 Special). However, our travel plans have changed where instead of using the 4 day pass over a 4 day period, I will like to use the 4 day pass over a 5 day period. Is that possible? If yes, how do I go about making that change. I will be happy to pay the price difference between the Swiss Pass and the Swiss Flexi Pass.

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