To add a bit more context to Frank's answer: adding a column to a table will work just fine today, but the data in that column won't propagate through to Materialize. You'll continue to see the latest inserts, updates, and deletes, though. The new column can be incorporated by re-creating the source, though this does require re-reading the entire table from the beginning at the moment.
Removing a nullable column will also work fine along the same lines: that column will simply be filled in with NULL in Materialize for all new data. Removing a non-nullable column is backwards-incompatible, of course, and so Materialize will be unable to process any additional events for that topic, and so you'd have to recreate the source and restart from the beginning.
