Thanks again..
I didn't realise that the file would be overwritten, should have read the doc more carefully.
I used meld to compare files when upgrading from 1.5.7c after removing any add-ons, but then I decided to do a fresh installation of 2.1.0 in a VM and apply bootstrap before importing data, images and text to the pages I had changed, then apply add-ons one at a time, taking snapshots before each, to see if there were any problems.
I had not actually made many changes, and the only add-ons I wanted to add were invoice payment and catalog invoice. (I had installed edit orders and zx-slideshow but I deemed them no longer needed). These days I tend to avoid coding, as my ageing brain struggles with it.:)