At this time we are very busy excuse me, I’ ll try to explain briefly, however Isn’t easy. The best case to avoid change band is to put GSM in the same band, then you will have an easy planning in GSM, however it depends on your percetages of penetration of each technology. If you have a mature UMTS & LTE networks its better to have limited coverage because you can contol better tbe interference at higher ftequency because you have less propagation than in lower frequencies, however you have better penetration because of wavelength. That are tbe base, however the costs play another point of view, you’ll need less sites with 900 Mhz to cover the same area.
If are with the situation that you told us there is no opportunities selecting equipmets and you need to work as you told us, i.e. multiband cells in GSM, by the way you must to work with multibands and multitechnology.
If you need something more specific tell us but give us some more details. Besr regards,