No hands on experience with the units you mention, but data migration is something that is dealt with in my job regularly.
I assume that each of the vendors has a PC based tool to manage waypoints, have a look at each and find a common format, download your waypoints from the Garmin to Mapsource or Homeport and export it he common format.
Then use the sported file as an import to the other software for the Lowrance unit and upload to the unit or an SD Card.
I have done this previously with TomTom to Garmin with success. From a quick google
https://www.google.com.au/search?q=l...AsKJmwWRvYHwCQ
It would seem that .GPX is the format for the transfer. EasyGPS is a free software app that might do the job.
Hope this helps.