I can create and update PushTopics without issue until the limit is reached. Once the limit is reached, I can't update existing topics, the following error is thrown:
Update failed. First exception on row 0 with id xxx; first error: LIMIT_EXCEEDED, exceeded the maximum allowed number of push topics: 
[note - xxx == Id of the PushTopic]
I am not inserting or upserting - just updating the query on a single topic.
If the org limit is 20 topics, everything works perfectly up until 19 topics. At 20 topics this error is thrown.