Modifying the CDT changes will that effect existing instances in production after deploying the CDT changes?

Hi,

If you add new columns this won't impact in-flight instance as long as certain conditions met

• These columns should accept null values
• You are not using this new columns in your expression/interface rules to determine some business logic, which will impact on workflow decisions

The in-flight instance will impact if any of the following changes are done your table

• Deleting columns
• Changing data type of column
• Reducing the size of column

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.