Plato's careful composition...

..of his works was, of course, noted in antiquity.  Most famously, there is this story, told in Dion. Hal. Comp. 25.209-18 (cf. DL 3.37):

ὁ δὲ
Πλάτων τοὺς ἑαυτοῦ διαλόγους κτενίζων καὶ βοστρυ- (210)
χίζων καὶ πάντα τρόπον ἀναπλέκων οὐ διέλειπεν
ὀγδοήκοντα γεγονὼς ἔτη· πᾶσι γὰρ δήπου τοῖς φιλο-
λόγοις γνώριμα τὰ περὶ τῆς φιλοπονίας τἀνδρὸς ἱστο-
ρούμενα τά τε ἄλλα καὶ δὴ καὶ τὰ περὶ τὴν δέλτον,
ἣν τελευτήσαντος αὐτοῦ λέγουσιν εὑρεθῆναι ποικίλως (215)
μετακειμένην τὴν ἀρχὴν τῆς Πολιτείας ἔχουσαν τήνδε
‘Κατέβην χθὲς εἰς Πειραιᾶ μετὰ Γλαύκωνος τοῦ Ἀρί-
στωνος’.

 But Plato did not cease, when eighty years old, to comb and curl his dialogues and reshape them in every way. Surely every scholar is acquainted with the stories of Plato's passion for taking pains, especially that of the tablet which they say was found after his death with the beginning of the Republic ("I went down yesterday to the Piraeus together with Glaucon the son of Ariston") arranged in elaborately varying orders. (trans. W. Rhys Roberts).