OMG it worked! And I scoured the whole globe for a solution! I had noticed the"Processor" menu choice in the past, but couldn't figure out why would anyone select the 1280, so that it must default to 2560 processor. But no, it sticks to 1280 when changing boards, and when the board is changed back to 2560, processor remains 1280.
I did not consciously set it to 1280. Must have accidentally changed processors, without realizing it was set to 1280. I even noticed this in the avrdude command string (-patmega1280) and changed it to -patmega2560. But it did not connect for some reason. I had no idea this was a user menu choice.
Thank you so much in0. That saved me a lot of grief. I was forced to IDE 2.0 because of this, and guess what, I like it more.
Thanks again.