Hi Justin!
Sorry for the long response delay. There isn't currently a built-in system for updating an existing bundle in-place. though I could certainly imagine an API for doing so in the future.
Probably your best bet right now would be to make your bundle writer try to load the data written by a previous `ingest` call. You can get your previously written data by doing something like:
>>> from zipline.data.bundles import load
>>> bundle_data = load('my-bundle-name')
By default, that should give you a BundleData object containing most recently-written version of the bundle named 'my-bundle-data'. `load` also takes an optional `timestamp` argument, which you can use to request data from a specific ingestion. The BundleData object is a namedtuple with `asset_finder`, `equity_minute_bar_reader`, `daily_bar_reader`, and `adjustment_reader` as attributes, which you can use to get your previously-written data.
Hope that helps,
-Scott