I think Tertullian's heretical adversaries had the reading found in our Arabic Diatessaron which goes:
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 cf. Borleffs J.G.Ph., CCL 2 (1954), (p.1002, l.38) BP1]
While the text does not specify in any way shape or form that he is citing the Diatessaronic per se reading it is obvious - at least to me that is - 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). N'est ce pas? Or is there something wrong in my thinking?