All jokes aside, BGA can be manually soldered; desoldering the old one is probably the harder part. Yes, reflow processes, but those can be done by hand with comparatively cheap equipment. It's not easy, though, and you would need to find the new chip somewhere...

I can see it now - N810 for sale, useable as door stop of for parts!

As for "soldering" BGA - can be done using a tuned infrared lamp. That lamp needs some really good calibration. If you search the net, you'll find instructions.
You'll also need something to remove the searing hot (300 °C or so) chip with before solder solidifies again, or place it chip down.
Installing new chip needs very careful alignment. Gravity should press strong enough to solder it in correctly.

I'm not mad enough yet to try that mod.

TA-t3 pats his N800 which came with two real, full-size SD slots! Hee hee..