We just updated our stream analytics query to add new columns (no other changes). However, all our old data is no longer accessible? Has anyone else experienced this. I would expect to keep the old data, and see new columns populated for new data.
Don't know if you ever got an answer to your post. I had the same experience and found Power BI Service does not like the schema of the dataset to change. I would get errors related to unable to visualize fields. The only way I got around this was to stop the stream, rename the output dataset, remove the existing dataset in Power BI, then turn back on the stream -- which, as you mentioned, renders the old data inaccessible since you're now using a new streaming dataset.
If you found a way around this, please post it here.