I'm a norstar installer but have a customer that has a Panasonic KX-TD 816C , Can I just replace here old KXT 7431 pnone with a KXDT 343 without having any programming changes and could I go back to the old phone without loosing any programming TIA

That phone wont work on the td systems. Will have to be a 74xx or older to work

You can swap a 74xx to any 72xx or 74xx phone without any programming changes

thanks that's all I need to know