I have no idea what “scribal change” means unless you are referring to copying errors, which were not uncommon but we have so many manuscripts of NT that they are easy to spot. 95% of all variants among manuscripts are spelling differences and word order difference - which is easy to occur with Greek, the meaning is the same. In Koine Greek there are at least a dozen different ways you could correctly write “ John loves Mary.” Most of the other variants are scribes accidently skipping a line and leaving it out or mistaking someone’s handwritten margin comments as part of the text and including it in the copy as part of the scripture. We can be confident that 99% of what you read today in a good translation is what was written by the author.
I'm not talking about copyist errors. I mean conscious changes made by the scribe where either the text says one thing and is changed to say something else or where the scribe makes up an entirely new piece of content not in the original and tries to pass his change off as part of the original text.
An example of the first type would be changing "Mary" (of Bethany) to "the sisters" with the accompanying verbs scratched out and switched from singular to plural in the Book of John. I wouldn't say that is nefarious, but it is a conscious change and not a dropped letter or some other kind of goof. It's obvious that the scribe thought about it first and then changed the text.
The second type is the wholesale insertion of new content like the second ending of Mark or the Johannine Comma. Both of these are attempts at pious fraud.