Change cite keys in bib file, keeping backwards compatibility

Question

I have a large .bib file which I always use for biblatex. To keep things more tidy, I recently decided on a systematic way to name the cite keys. I would like to also rename the old entries to follow the new system, but I don't want to break citations in old files I have created that cite using the old keys.

Is there some way to have more than one key for an entry, or to have some sort of metafile that reroutes the old keys to the new ones, without having to change anything (or maybe just one line like "\include{metafile}") in the old files?

Some of my old files still use bibtex, so a solution that also works in bibtex is preferred.

So I have a .bib file

@article{somekey, 
   author = "author"
 }

I want to change this to

@article{newkey, 
   author = "author"
 }

But I want that

\cite{somekey}

still works.

Answer 1

Just a short summary of the answers to the linked questions:

with biblatex:

@article{key,
   ids = {otherkey, anotherkey},
   ....
}

with bibtex:

@article{newkey,
   crossref = {oldkey}
}

@article{oldkey,
   ...
}

Another way is to put stuff in the preamble and modify the \cite command, but since I asked specifically about ways that wouldn't need any modification in the tex file, I didn't include them here.