Yes, it is possible to extend the target duration of your A/B experiment after it has been set up. Changing the target experiment length in the advanced settings will not result in data loss or change the assignment to Control/Test. The target experiment length is primarily used for tracking progress against your experiment's duration. It does not impact the data collected or the group assignments within the experiment.
By default, Statsig will compute Pulse Results (i.e., metric lifts) for your experiment for the first 90 days. If you need to extend beyond this, you will be able to do so as you approach the 90 days cap. Please note that extending the duration of your experiment does not affect the data collected or the group assignments within the experiment.