Moving attributes from a design on to an interface

It is possible when creating a design to think that certain attributes only apply to that design, so it is not worth putting them on an interface. However further down the line, things can change and you regret that they aren't on an interface. There are also designs that were created prior to us being able to create interfaces, that would benefit from being tidied up.

I don't know if what I am asking is possible, but it would be good if we could retrospectively add attributes to an interface. Currently we would have to create a new attribute on an interface and add it to the design, then update that attribute for existing records, then hide the old attribute (or clear it and then delete). None of that looks good in the logs if we have court cases where we have to provide the history of an asset.

To go one step further, it would be useful if two attributes could be merged into one, but only if only one of them was populated. That would help tidy things up and make reporting easier.

I appreciate that if it is possible it would require thought as to the circumstances it could happen, but it would be really useful.