A nominative, accusative, or vocative antecedent, when incorporated, usually conforms to the case of the relative.
εἰ ἔστιν, ἣν σὺ πρότερον ἔλεγες ἀρετήν, ἀληθής (for ἔστιν ἡ ἀρετὴ ἀληθής, ἣν)
a. An accusative antecedent is incorporated in the accusative when the verb of the relative clause takes the accusative. Thus, οὐκ ἀπεκρύπτετο ἣν εἶχε γνώμην (for τὴν γνώμην ἣν)
b. An accusative antecedent may be incorporated as nominative, genitive, or dative, e.g. εἴ τινα ὁρῴη . . . κατασκευάζοντα ἧς ἄρχοι χώρα_ς (for τὴν χώρα_ν ἧς ἄρχοι)
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].