You can deregister all of the handsets and then register each one again for the LS6475-3. Make sure that all of the handsets are out of the base and all chargers and that the phone and all handsets are in standby. Then press and hold Find Handset on the base. When the In Use light begins to blink, release the Find Handset button. During the seven seconds that the In Use light is blinking, press and release the Find Handset button. Wait for the deregistration process to complete (about 10 seconds).
If the process fails, you may need to reset the base. You can do this by unplugging the AC power cord to the base or by picking up a registered handset and pressing Talk/Flash. Then you can try again. These directions are on page 5 (page 8 of the PDF) of the manual for the LS6405 handset. The direct link to this manual is https://cdn-media.vtechphones.com/media/p/document2/products/%7BD6DAD9B0-4C1A-4882-93D9-04124A59B629%7D/LS6405_CIB_V5_20160805.pdf
. Then check that all of the handsets are in standby and not on any charger before attempting to deregister the handsets again.
You can register 12 handsets to the LS6475 base, according to page 31 of that manual for the main unit: https://cdn-media.vtechphones.com/media/p/document2/products/%7BA37C95FC-053D-4D34-B60A-EA5ED42B056C%7D/LS6475-3_UM_4_925.pdf
. If you use the cordless headsets, you can register 10 handsets and 2 headsets.
These manuals can be found through the product page: https://www.vtechphones.com/pd/1603/LS6475-3-2-Handset-Answering-System-with-Cordless-Headset
. The base manual is in the section under the Support & Manuals tab. The handset manual can be found from the Accessories tab. Then click on the Support & Manuals tab after clicking the link for the Handset.
I hope this helps.