Re: changes to thumbnail handling between IM 6.49 and IM 6.5.9?

I don't know if this is relevant or not. Anthony would be best to 
answer this. But at some point (not sure what release) some of the 
resize filters also used for thumbnail went through some changes and 
new additions. I don't know if the default filter changed. But you 
can try using -filter with various filters and expert settings to 
recreate your old looking images.

see http://www.imagemagick.org/Usage/resize/#filter

you can check the changelog to see if you can find when this occurred.

see http://www.imagemagick.org/script/changelog.php