I am not aware of any add-on that will do this kind of transformation, but if you are comfortable working on your EE4 database directly using SQL it is certainly possible to do that way.
I had to do this recently sorting out an EE2 site being migrated to EE4. For that work I was pulling entries from channels and writing them to fluid fields (including grid fields). Worked OK. Didn't try and migrate relationship fields - but imagine that they are doable too.
A detailed description of what to do is beyond scope of this kind of space, but simplistically:
- Use EE to create the fluid fields you need
- Put one unique fake entry information in each field
- Use something like Sequel Pro to find the definition information for each field created (e.g. the channel field / grid number, and the position of the field in the fluid field register)
- For each field, generate a list of the entry_id's of the entries that you want to migrate and that have a value in the field
- Construct a custom SQL query to copy the content of the migrating fields to the new fluid field entry - you will need to make sure you increment some counters in the fluid field register to do this correctly
- (side note - I wrote a set of EE templates to automatically generate the required SQL queries - saved a lot of time)
- test / delete the old fields.
This is a non-trivial activity for you by sound of it, but once you get hang of how it works it should be manageable.
Usual words of caution - back everything up, and do this work on a development server... it is easy to break things.
Of course if you want someone else to do it for you... drop me a line via my website :)
