[Koha] Batch import-updating of existing biblios

[hansbkk at gmail.com]

This isn't just a question on Koha's capabilities - I'm also seeking
general advice from a workflow/process POV.

Say I have a set of biblio records that are just the way I want them,
in the sense of the accuracy of the existing data in the currently
populated fields.

And I would like to "add value" to them later on - say I find a MARC
source that assigns subject headings in a way that I like, or adds
links to cover images, etc. looking to selectively overlay/insert a
subset of the new records into my existing ones.

How does Koha handle this?

Or would I be better off exporting my current records to MARC, using
an external MARC editing tool to handle the merge and then re-import?
Can anyone suggest tools for me to look at? I'm aware of biblios.net
and MarcEdit so far.

Obviously being able to do this in batch mode with reasonable matching
rules would be ideal rather than one at a time, as long as I can
review the results before "committing" the update.

Or if it's really better to do this one record at a time, does that
change the recommendation as to using Koha for the process?

And finally, what if there are certain field values that I do want
overwritten in addition to the new ones being brought in, but some
fields shouldn't be touched at all?

Thanks in advance. . .