This seems like a mine field...
Tried to convert most Categories to Tags. Selected the ones to convert and left 4 to keep as categories.
The conversion was not complete, and some categories remain in the category list. However now if I try to run the conversion again, only my original remaining 4 categories are presented for conversion, but not the additional ones that were not converted the first time round.
Also getting an error in Category Cloud Widget (which worked fine until the conversion)
"Warning: Division by zero in /homepages/26/d178196719/htdocs/23lsb/wp-content/plugins/category-cloud.php on line 169"
Any ideas what has gone wrong?