Migrating from 'lawn' to 'bushy' blobstorage - POSKeyError, no blob file

This concerns an upgrade to 4.3.7 from a 3.2.2 (buildout based) site.
The progresion has been 3.2.2 -> 3.3.5 -> 4.0.8 -> 4.3.7

It appears that when I went from 3.3.5 to 4.0.8, and the file objects in Data.fs were moved to blobs, the default layout for the blobstorage was 'lawn'.

Then when I did the upgrade from 4.0.8 to 4.3.7, there is this notice in the server log:

2015-12-18T13:19:52 (22356) The lawn blob directory layout is deprecated due to scalability issues on some file systems, please consider migrating to the bushy layout.

The 4.3.7 site is working fine with 'lawn' layout (Debian linux server). However per the notice, it seems it would be prudent to migrate to 'bushy.' Found this:

Thank you Ross Patterson for clear directions!
However after following the above instructions, and a migration apparently happening - I can look in the blobstorage directory for the site and see what looks like the new arrangement of blobstorage - I get errors on all file objects, one way or another. Images are now missing/broken on pages; files such as PDFs etc. return an error with traceback always ending in:
Module zope.tales.expressions, line 217, in call
Module Products.PageTemplates.Expressions, line 155, in _eval
Module Products.PageTemplates.Expressions, line 117, in render
Module plone.app.blob.field, line 129, in get_size
Module plone.app.blob.utils, line 52, in openBlob
Module ZODB.Connection, line 860, in setstate
Module ZODB.Connection, line 922, in _setstate
Module ZEO.ClientStorage, line 1020, in loadBlob
POSKeyError: 'No blob file'

Need help with what next to troubleshoot this situation.
I've found some online posts about the POSKeyError but they are all about corrupted situations of various kinds, which I am pretty sure I don't have.
I've not found anything yet relevant to the migration script producing a 'bushy' migration result that is broken like this.
Any leads appreciated...

John S.

Ugh, that seems scary to me (changing layout as part of an upgrade). Do you have to do the layout change now? I would be inclined to do the upgrade and separately experiment with the layout change.

To clarify - the upgrade is done. The site is running on 4.3.7, with 'lawn' blob file layout.
It was after finishing the upgrade of the site that I found the notice in the server log that 'lawn' blob file structure is deprecated and one is encouraged to migrate to 'bushy'.

So I'm trying to do that - my first attempt, following the clear instructions and using the migration script resulted in a fail, with the POSKeyError for all blobs. Need some help with what to try next - how to troubleshoot the result of the migration, or the migration itself, or anything anyone knows about what might make the migration work.

What I was saying above is that I hope that you are not dead in the water at this point, ie. you're attempting the layout change in a copy of your installation, not in your production installation.

But on the particulars of the layout change problem you've run into, I'm afraid I can't help.