in our days you may study in 3GPP and system documentation the topic ABSOLUTE CELL PRIORITY. To the LTE, UMTS GSM900/1800 layers an absolute cell priority can be assigned. If the mobile has LTE capability, it can be guided during handover with highest priority to LTE or with lower priority to wcdma, gsm 1800, gsm 900. Personally, i would asign the lowest priority to gsm 900. In the old days, the featuter HCS hierarchical cell structure and Re-Direct have been used to shift as much traffic as possible from GSM 900 to GSM 1800.