The Gospel of Marcion had a known Diatessaronic variant of Matthew 17:2

I am credited in an article of Baarda's somewhere as getting him to look at the Transfiguration in the Arabic text. There are two interesting variants. The first was the one he spent most time on - the strange editorial reference to the existence of another version of the Transfiguration identifying it as taking place after the Passion (i.e. "And they thought that the time of his decease which was to be accomplished at Jerusalem was come.") Baarda considered and rejected my understanding that this demonstrated knowledge and a subsequent rejection of a heretical version of the story after the resurrection (as in the Coptic Apocalypse of Peter and has been theorized by Teeple and many others).

To be honest, I was unhappy with Baarda's conclusions and sucked out on helping him proof read the article.  But until now I didn't have a comeback because quite honestly Baarda is like brilliant and a master of every language under the sun and I am just some blogger who is rather stupid at times.   Well I didn't have much to go on at the time until I started doing this seemingly moronic 'super-gospel' exercise.  But now I found something which I think proves my original assumptions that Tertullian's heretical adversaries had the reading found in our Arabic Diatessaron.  It develops from the second significant variant in the passage just before the 'after the time of decease in Jerusalem).  We read in the Arabic Diatessaron:

And while they were praying, Jesus changed, and became after the fashion of another person; and his face shone like the sun, and his raiment was very white like the snow, and as the light of lightning, so that nothing on earth can whiten like it. And there appeared unto him Moses and Elijah talking to Jesus. And they thought that the time of his decease which was to be accomplished at Jerusalem was come. 

Compare this reading with what is said in Tertullian De resurrectione mortuorum:

The Lord, again, in the retirement of the mount, had changed His raiment for a robe of light; but He still retained features which Peter could recognise. In that same scene Moses also and Elias gave proof that the same condition of bodily existence may continue even in glory-the one in the likeness of a flesh which he had not yet recovered, the other in the reality of one which he had not yet put off. It was as full of this splendid example that Paul said: "Who shall change our vile body, that it may be fashioned like unto His glorious body." But if you maintain that a transfiguration and a conversion amounts to the annihilation of any substance, then it follows that "Saul, when changed into another man," passed away from his own bodily substance; and that Satan himself, when "transformed into an angel of light," loses his own proper character. Such is not my opinion. So likewise changes, conversions and reformations will necessarily take place to bring about the resurrection, but the substance of the flesh will still be preserved safe. [De resurrectione mortuorum 55 § 10 BORLEFFS J.G.Ph., CCL 2 (1954), (p.1002, l.38) BP1] 

While the text does not cite the Diatessaronic reading it is obvious (at least to me) that Tertullian (or his source) is combatting heretics who use the Transfiguration narrative as proof that we change into another person when we are resurrected. Hence Tertullian's citation of the canonical texts where the offending line 'changed into another person' does not appear. The implication nevertheless is that the heretics must have had the reading prior to Tertullian's citation and thus the reading existed in the late second century (= Tatian).

Going back to the reference to the apostles wrongly thinking that the Transfiguration happened after the time of his decease at Jerusalem.  I again think - as I originally suggested to Baarda - that this means the editor of the Arabic Diatessaron was correcting the original 'Diatessaronic-gospel' of Tatian or some other second century figure explicitly identified as the gospel written by Peter and the apostles (see above).  It's identification as a variant of Matthew 17:2 is confirmed by A Hobson The Diatessaron of Tatian and the Synoptic problem p. 66