We should provide a way to convert existing tables to be transactional. The main problem is the existing data in the tables, which will have been written with normal millisecond timestamps, instead of transaction IDs. If TTL is enabled on the table, for example, all of the existing data could be erroneously TTL'd due to the multiplier used for the timestamp portion of transaction IDs.
To handle the TTL case, we could apply a simple heuristic when inspecting cell timestamps. If the timestamp is less that the current time in milliseconds, times a small multiplier to guard against clock skew, then TTL is applied based on milliseconds. Otherwise TTL is applied as currently handled, using effectively nanosecond precision.