Field data type conflict

I have nested field type (text, long). After upgrading to v 8.6, a conflict happened in this field. Below is the message I got:
"The type of the (field name) field changes across indices and might not be available for search, visualizations, and other analysis."
How can I resolve this conflict issue without losing my data?

It'd be helpful if you could share the mappings of the conflicting indices.