The verse is found in Mahābhārata, 13.24.70:

vedavikrayiṇaś caiva vedānāṃ caiva dūṣakāḥ |

vedānāṃ lekhakāś caiva te vai nirayagāminaḥ ||

The first and the last group are relatively easy to be identified (although: what is meant by “selling” the Vedas? Making money with Vedic rites?), whereas the vedāṇāṃ lekhakāḥ are more elusive. I would have thought of them as “offenders of the Vedas”, a category which would have been by the way ample enough to accommodate also the other two groups.
However, Petra Kieffer-Pülz pointed out a somehow parallel passage in a Pāli source, where the fact of altering (scil. purposefully) the wording of an Aṭṭhakathā is condemned (the passage is dealt with in her Verlorene Gaṇṭhipadas, II, pp. 947–958). Thus, it seems that the author of this Mahābhārata verse was aware of attempts to modify the text of the Vedas and was keen to preserve its immutability.

Do you know of other evidences of the custom of altering texts?

I am grateful to Petra for pointing out the MBh verse and her discussion of a similar issue and for discussing them with me.