I am changing phone depend on the hardware, I change depend the Screen size, processor and GPU.
For example:
My Xperia Neo V is 3.7 inch , Snapdragon MSM 8255 1GHz Single Core , 512MB ram , Adreno 205
Then
I change Xperia Ion, 4.6 inch , Snapdragon MSM 8260 1.5GHz Dual Core , 1GB ram , Adreno 220
And Then
I change Xperia Z Ultra, 6.44 inch , Snapdragon 800 MSM8974 2.2GHz Quad Core , 2GB ram , Adreno 330
As you can see,
Processor: Single Core >> Dual Core >> Quad Core
RAM: 512MB >> 1GB >> 2GB
Actually I don't think why alot people change phone without reading the spec. Some people is using Dual Core and 1GB ram phone and he/she still change to New Dual Core and 1GB ram phone. I just change phone like this as I see that's few parts upgrade like Processor , RAM , GPU and Screen size
I changed from Note 2 quad core amoled to dual core slcd.