I would suggest you leave the zero offset at 2.5V to get maximum ADC range and use switched amplifiers or attenuators to scale the signal as required. I do not understand why you want to change the offset to measure a wider input range as it does not improve accuracy.
Commercial systems work by trying to maintain the reading within range by "auto ranging", basically, if the reading is below a threshold, the amplifier gain is increased and if the reading is above a certain threshold the gain is reduced. If you do this to maintain a measurement in the region 0f 80% - 90% of full scale it should work well and still be bipolar.
Brian.