Speaking from experience, we do these stuff regularly at work. Here some random comments:

- It is possible to replace the BGA device. The success rate is quite good but we don't do this for operational boards, we can not trust it for long term and reliable operation. We do this only for prototypes that had a mistake. The high cost can be justified there since we cannot wait 2-3 months for building another proto.

- usually we don't reuse devices. It is very difficult not to destroy the chip when it is removed. The cost of the reballing is high and in the end for most devices does cost the same as new.

- no-one uses x-ray to aligh the pins. We use the xray machine for the pre-series production to check the factory and how clean was their job.

If I were you, I wouldn't do this mod unless there was something already broken.
If you going to do it, ask the company to buy a new memory for you. Don't re-use from the other phone.

Hope it helps