Yes you can change the required field at any point. It will not affect the existing documents until after someone tries to edit them, only at this point Sharepoint will force user to enter data for required fields.
If you have some pattern you can follow you could write PowerShell script to loop over the items and update the metadata (sometimes you can have indication of the metadata in the document name, sometimes info in different field may inform what other metadata should be)
Changing the boolean type to column is more problematic. Not sure if even supported after the creation. But you can deal with it differently. Create new column as choice enter whatever values you want (I guess Yes/No will still be an option) Then use either PowerShell/Designer Workflow/Flow to copy the values from the boolean to choice column. After that validate that everything is as you expect it, if yes then you can delete that boolean column.