Context
We use fixVersions for two purposes:
- For roadmap planning of (functional) releases. Every epic & story in an epic is assigned the fixVersion of the release they will be part of. This allows us to use the Roadmap functionality to quickly see what is left for that release.
- For technical release notes, when a story is implemented and the code is merged a technical fixVersion is assigned to track the technical releases and generate release notes.
We want to make sure that if Epic A is assigned fixVersion X, that all tickets in that Epic also have fixVersion X. When the Epic changes fixVersion to Y, we want all tickets in that Epic to have fixVersion X removed and Y added.
Question
I'm trying to create an automation rule that when the fixVersion of an Epic changes, removed the old fixVersion from the children and adds the new fixVersion to the children. I don't use the 'copy fixVersion from parent' because I don't want to overwrite the technical fixVersions that have been additionally assigned to the stories.
Following the documentation I found here and here, here's my last attempt:

1. When the fixVersion changes => This should give me access to {{fieldChange}}
The changed field value will be available anywhere smart values are supported using the {{fieldChange}} substitution. Use {{fieldChange.fromString}} and {{fieldChange.toString}} to access display values and {{fieldChange.from}} and {{fieldChange.to}} to access raw values (for a select field for example). {{fieldChange}} will only contain the first changed value. If multiple values are changed (e.g. when setting multiple Fix Versions) then you can iterate over these using the {{#changelog.fixVersion}}{{toString}}{{/changelog.fixVersion}} expression.
2. Using advanced fields it should be possible to create a json in which you can specify which fixVersions need to be added to the multiple value list and which should be removed. I tried different notations, but each time I get the error "invalid json" when the automation rule is triggered.
Does anyone know how I can remove the old values of the fieldChange and add the new values of the fieldChange for this multi-valued field fixVersions?
Thanks