Not the guy you're asking, but I recently said "portico" when I meant to say "patio." I mixed up the two words because they both start with a P and have something to do with architecture. I think this is the kind of thing he meant by "replacement," and it's been happening to me (55) more frequently in recent years.

Yup, precisely this. I'm inclined to think it may be more sleep deprivation driven than anything else, as I find myself doing it intermittently - and the odd thing is I'll proof what I've written thrice and I'll still mis-read incorrect words as what I had intended rather than what I wrote.

Definitely didn't do this in the past, I find it disquieting, but it's hardly insurmountable!