thanks mark - the solution was there indeed!
Someone (!) had named my current LP a 'Logic Pro 9', and as soon as I changed it to 'Logic Pro', the updater worked.
Interestingly enough, it has left the old LP in the same folder as 'Logic Pro 9.0.0'.
How tempting it is to rename the new version 'Logic Pro 9.1.0' - it would be to my advantage to do so, since any eyeball or spotlight search under 'logic' will yield at least 2 apps. Plus it is easier to locate for my dock alias. But no, I must leave the heart of my music system undistinguished ( beyond of course that visible when I view info, a pernicious operation that reminds me of DOS)