The installer trawls though the defines in the existing admin configure.php file to check all is well (existing files in the right place, existing database connections working etc.).
However I note it uses the first instance of the constant it finds, whether in use or commented out (for whatever development reason).
So, if the commented define is first in line, and it is not correct for that installation, it doesn't check out and so no Upgrade is offered.
Not asking for this oddity to be taken into account, but it caught me out (for a good while).

"Why multiple defines" you ask...well I have the development sites files/all my files synced/duplicated across various pc's so I can work at whichever is convenient and although the local pc's virtual hosts definitions can be identical, their filesystem location may not be, so DIR_FS_CATALOG may vary.
So I have this in the file, so it works in any of them.
$dir1 = 'D:/Documents/some location.../public_html/tienda_155/';
$dir2 = 'D:/My Documents/some other location.../public_html/tienda_155/';
etc...
if ( is_dir($dir1) ) {
define('DIR_FS_CATALOG', $dir1);
} elseif ( is_dir($dir2) ) {
define('DIR_FS_CATALOG', $dir2);
etc...
}
The installer was picking up $dir1 as the directory.

Still, I've learnt how the installer works now!