While a merge between branches may work, with a clean checkout, it failed for me to recognise the renames I had on my checkout at the time of running "fossil update".

Committing, resulting in a fork of the branch, and then merging the leaves with a clean checkout, worked as a trivial merge.

Luckily 'fossil update' invites to undo, and the harm is then minimum.

Merge to the checkout (on update) fails, if with renames