Ok, found a workaround for Humminbird releases 5.840 or 6.180, both with Waypoint Management System.
Bobby from GpsLakeMaps sent me the Lowrance version of Fork which is a .USR file for testing.
Used HumminbirdPC to OPEN this .USR file, prompted me to continue and convert to GPX format, answered YES, then a new HumminbirdPC file appeared in the list of opened files.
Then you upload this new HumminbirdPC file onto your SD card and load successfully into your unit.
Confuses me why one has to convert-Lowrance-to-Humminbird for HumminbirdPC to force the GPX file-record reformat to the new data record format.
Since the test was successful, I used the GPSBABLE open source program to convert-to-Lowrance on all my other Boatlane and GpsLakeMap files and it works also.
New GPX file also seems to be backward compatible, but I would save my old one.