Determination of time-to-event in time series

Hi everyone,
I'm going to make a kaplan-meier plot and have met some difficulties on the way. Every User in my dataframe have several rows at different timepoints. At every timepoint correspond to a new measurement of a biomarker and the corresponding level (divided into three groups). I need to determine whenever patients "shift group" for the first time in the time serie - they can shift more than once, but only the first change is of interest. Baselinedate = 0 indicate the first measurement and is used as the reference.

I hope you can help me. Thank you in advance!

Best regards

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