From Meyer's NT Commentary: epexegesis: (namely) the redemption of our body from all the defects of its earthly condition; through which redemption it shall be glorified into the σῶμα ἄφθαρτον similar to the glorified body of Christ (Php 3:21; 2 Corinthians 5:2 ff.; 1 Corinthians 15:51), or shall be raised up as such, in case of our not surviving till the Parousia (1 Corinthians 15:42 ff.). So, in substance (ΤΟῦ ΣΏΜ. as gen. subj.), Chrysostom and other Fathers (in Suicer, Thes. I. p. 463), Beza, Grotius, Estius, Cornelius a Lapide, and most modern expositors. On the other hand, Erasmus, Clericus, and others, including Reiche, Fritzsche, Krehl, and Ewald, take it as: redemption from the body. This is linguistically admissible (Hebrews 9:15); we should thus have to refer it, not to death, but to deliverance from this earthly body through the reception of the immortal and glorious body at the Parousia, 1 Corinthians 15:51. But in that case Paul must have added to τοῦ σώματ. ἡμῶν a qualitative more precise definition, as in Php 3:21 Remark.
See also Hans Lietzmann, An die Romer, page 85.