Agreed, I would strongly recommend retuning. But why the switch? If I'm understanding correctly you'd be going from a much newer technology to an older technology?
I'm guessing you're using our 900cc high impedance injectors which flow the same as injectors referred to as 1000s from other companies. (These are ours: http://fuelinjectorclinic.com/d-b-h/IS115-0900H