The second-quantised description of the electromagnetic field in terms of oscillators holds in QED as well. The part that is modified is the single particle description of charged particles. In other words, (virtual and real) pair-creation is permitted in QED. So for energy scales less than $2mc^2$, where pair-creation is not possible or suppressed, one can work with a fixed number of charged particles.

(An intermediate step to QED is to go from the non-relativistic Schrodinger equation to the Dirac (or Klein-Gordon) equation. The breakdown of this description is illustrated by the Klein paradox.)

The second-quantised description of the electromagnetic field in terms of oscillators holds in QED as well. The part that is modified is the single particle description of charged particles. In other words, (virtual and real) pair-creation is permitted in QED. So for energy scales less than $2mc^2$ as well as low intensities (see Schwinger limit), where pair-creation is not possible or suppressed, one can work with a fixed number of charged particles.

(An intermediate step to QED is to go from the non-relativistic Schrodinger equation to the Dirac (or Klein-Gordon) equation. The breakdown of this description is illustrated by the Klein paradox.)

(Edited to incorporate @curiousone's comment.)