You can simply access those nodes inside WCMUsePojo or Model and write a function which converts nodes data into JSON and return for further processing.
The idea is let AEM stores data in node structure but if you already have written your logic based on JSON, just put few lines of code to convert Node to JSON, if you are writing new logic then go ahead with node structure no need to convert.
Converting Node to JSOn at backend is super easy as compared to actual store values in json in node.
I am not sure if this is going to solve the problem. I want to convert the actual data.
The scenario is, I had a coral 2 component and there are multi-fields saved in Json format. Now as coral 2 component is upgraded to coral 3, the new multi-field values are getting saved as nodes. The upgraded component is fine as long as new multi-fields values are saved and displayed. The problem is with the previous multi-field data stored in Json. Can I covert those multi-field Json data to nodes so that upgraded component can work well with both new and old multi-fields?
If you have any other suggestion to deal with existing Json format multi-field with coral 3 component.