I had an interesting experience the other day. I have been using split keyboards for years, but only recently began using a Kinesis Advantage 360 Pro as my daily driver for work; I am so use to the layout now, that in instances where I have to use a non-split, or even my older splits, my fingers fail me entirely - like my brain forgot how to type, my thumbs eagerly searching for the thumb cluster that isn’t there.
My speed on row-staggered QWERTY non-split keyboards is pretty much the same as before. Maybe it’s because I switch to Colemak a few weeks after switching to split columnar keyboards. I think that they are just separate in my head now. Similar to how learning to use split keyboards didn’t erase my memory of how to ride a bike.