It is possible to export and import a project area process template from one Jazz/RTC instance to another. (could be done via the web or eclipse client)
This kind of template an be developed on a test/develop instance and after the export/import can be used for NEW project areas. In RTC the selection of a PA template is done during the creation of the PA (project area). There is no official command to replace a PA template in an already used PA.
If really necessary - but depending on the changes in the template - you can replace the underlying .xml file in the real PA. (could only be done via the eclipse client with copy and paste - a backup would be a good idea).
Another option is to use the shared process option. Means the used PA itself is created out of a template but than mapped to another PA which only reason is to provide the process. (make sense is a lot of PA should use the same process), Now the process only need to be changed once in the process provided PA.
about the impact: depending what you want to change:
easy: add more attributes -> attribute will be empty on old/existing artifacts
add new value to existing attribute - could be used in future
add an additional state - state could be used in future
complicated: change workflow, delete existing state -> artifacts with this old/deleted state need to set manually (or great scripts) to the new state
delete existing attribute values-> artifacts with this old/deleted values need to set manually (or great scripts) to the new value
Exporting and importing process templates
https://www.ibm.com/support/knowledgecenter/en/SSCP65_6.0.1/com.ibm.jazz.platform.doc/topics/t_export_import_process_templates.html
Migrating a project to a different process template
https://www.ibm.com/support/knowledgecenter/en/SSCP65_6.0.1/com.ibm.jazz.platform.doc/topics/t_migrate_dif_process.html
How to manage and customize the process template for your lifecycle project
https://jazz.net/library/article/1427
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