It’s finally a bug, the last answer of the support:
“Our Engineering team are definitely aware of this issue and we are looking into improvements for post migration processes”
To be able to delete the custom field and then replace it:
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Asana support has to unblock the wrong custom field, so that you can edit it
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then one strategy is to leverage the advanced search to find specific tasks with a specific value from the original field and then bulk-set the equivalent value in the new field.
OR
you can use @Bastien_Siebman 's tool to find all the project (you have access to) where the custom field is used: iDO Tools - Improve Asana with our tools and automations -
From here, once you have adjusted and consolidated the fields, you can then adjust the rules accordingly to be linked to the new field.
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Check also if Portfolios are using those fields
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Finally, you can delete the wrong field
Asana support can’t do this automatically, thus it’s a lot of manual checks.