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.
1. From 900 cell to 1800 cell same site:- You can activate DADLB(Direct access to desire layer/band) feature.
Check the value of NBL(if less then more traffic would transfer on 1800) for 1800 segment.
Check LAR and LER PARAMETER as well.
2.from 900 of congested cell of site to 900 less congested cell of other site :-
Enable direct retry,TRHO.
If lots of handover call then you can change also the BLT value of congested site as well.