your non-capable EGSM MS are screwed !!
Keeping all of them in the DCS band is a solution, and I don’t have any other idea in my mind… But they will experience call drops when leaving the DCS1800 cells. And “no network” in the rural area without DCS1800 coverage.
What is the penetration rate of the EGSM MS ? More than 5% ?
The best approach is to replace the old PGSM MS by newer EGSM MS. You could track them down by doing a a-interface trace.
Then, you need to involve Customer Relations / Marketing dpt, so they can offer new phones to those (old & very devoted) customers.