How to copy a solr index to another index with a different schema collapsing stored data?

["ristretto.rb" <ri...@gmail.com>]

Is it possible to copy stored index data from index to another, but
concatenating it as you go.

Suppose 2 categories A and B both with 20 docs, for a total of 40 docs
in the index.  The index has a stored field for the content from the
docs.

I want a new index with only two docs in it, one for A and one for B.
And it would have a stored field that is the sum of all the stored
data for the 20 docs of A and of B respectively.

So, then a query on this index will tell me give me a relevant list of
Categories?

Perhaps there's a solr query to get that data out, and then I can
handle concatenating it, and then indexing it in the new index.

I'm hoping I don't have to reindex all this data from scratch?  It has
taken weeks!

thanks
gene