Um, why? The device can't prefer 2.5g over 2g or the other way around. 2.5g modulations are only ever used if actual packet data needs to be used. At other times, the same command channels are used with both 2g and 2.5g.

If it's constantly "switching", it just means that at&t has a very patchy network with mixed ancient base stations and new basestations, and that you are in poor coverage if it's constantly changing between different cell towers..