I deal with Angel Eye bulbs all the time. Replacement is EASY, just a simple bulb swap.
BMW bulbs are expensive, you will pay at least 40 EUR per bulb.

Get the aftermarket bulbs, make sure they are at least 10W-20W though and they cost 30-50 EUR per pair. But don't be expecting miracles. They will slightly brighter than OEM ones, but not as bright as brand new BMW F models.
Let me know if you need more help on this matter.