Hi,
Discussion posts and replies are publicly visible
If you add new columns this won't impact in-flight instance as long as certain conditions met
The in-flight instance will impact if any of the following changes are done your table
Last I checked, running process instances will continue referring to the version of the CDT that existed when that instance launched. That said, be aware that any subprocesses will be using the new CDT version with added column(s), which you might need to account for in your design.