how to check if field does not exist in old CDT

The way I've handled this before is to pass in a new parameter to the form, i.e. ri!isNewVersion (or ri!hasFieldXYZ), where the process model is updated to pass a value of true() into the form, so that you can use in-form logic to tell whether you're in a new instance or not. The nomenclature is completely up to you but hopefully you get the picture.

Expanding on Mike's suggestion, what I usually do here is add a field to the CDT during the next update, integer value, named "version". Set this in the CDT at process initiation, I use a pv!version variable, set at 2 at this time and save into your CDT prior to any Write to DS (usually script task at the very beginning). When utilizing the pv! you can increment this value in the model any time you will be adding CDT fields or making changes that affect how your forms or sub processes will process going forward.

On your forms, utilize a local version variable with the definition:

local!version: index(ri!CDT,"version",1)

This will default to 1 if the version field is not present (legacy instances), or reflect 2 for newer instances. From there, you can implement logic in the form processing based on which version of your CDT is present, without manipulating any running instances, such as, if(local!version>1, show new fields, do nothing). This allows scalability on your future versioning as well.

Expanding on Mike's suggestion, what I usually do here is add a field to the CDT during the next update, integer value, named "version". Set this in the CDT at process initiation, I use a pv!version variable, set at 2 at this time and save into your CDT prior to any Write to DS (usually script task at the very beginning). When utilizing the pv! you can increment this value in the model any time you will be adding CDT fields or making changes that affect how your forms or sub processes will process going forward.

On your forms, utilize a local version variable with the definition:

local!version: index(ri!CDT,"version",1)

This will default to 1 if the version field is not present (legacy instances), or reflect 2 for newer instances. From there, you can implement logic in the form processing based on which version of your CDT is present, without manipulating any running instances, such as, if(local!version>1, show new fields, do nothing). This allows scalability on your future versioning as well.

Thanks for the suggestion. This seems like a possible solution but I don't feel comfortable hardcoding behaviors based on versions as we add fields around once a month.

No problem. Note since your process instance and CDT are essentially hard coded together based on their configuration at start time, you will need some similar approach here as there are no OOTB functions to cleanly determine field existence within a CDT. Utilizing "if(version > 1 .." will provide scalability for future versions without any additional changes to these fields, once configured.