Remember that the keys operate 3 different systems:
- EWS - immobilizing system
- FZV - central locking system
- DWA - anti-theft alarm siren system
EWS uses an encrypted ISN (individual serial number) that is coded to the car and only BMW has the information in a database. This information is communicated between the DME, key and CAS/FEM/BDC modules (on these later F & G series). This data is hard to retrieve (if even retrievable in the latest iterations).
The FZV & DWA (lock/unlock & arm/disarm) features of the keys used to be reprogrammable on other BMWs that operated on the same radio frequency...but I'm not sure how easy or simple this is anymore with the increased technology of the newer gen BMWs with comfort access etc. But even if it is doable...it is different than the immobilizing (EWS) engineered designs.
This is what makes used BMW keys almost impossible to reuse. The older E series BMWs can be repurposed if you have an AK90 set-up...which requires retrieving the EWS ISN data from the DME or EWS control module...then rewriting that info on a new transponder that is inside the remote key fob. But again, the new gen BMW keys that have comfort access have a higher degree of technology and the data is written to the CAS/FEM/BDC modules...and I'm not sure how a professional keysmith (or thief) goes about retrieving this data and writing it to a new transponder chip which would then need to be installed in an old (already used) BMW remote key fob.
The mechanical key can be purchased separately (see #5 in diagram)...so if you can find someone who can actually retrieve the encrypted data and write it to a new transponder...as well as fit it inside your old remote fob...you can order a new keyblade from BMW and get it already cut through them...or if you want to chance it with a local keysmith...you can buy a blank, then have them cut it. But I'd make d$#m sure the local keysmith knows for certain that they can do it...and if they screw up...its up to them to make it right or eat the cost of a screw-up/mistake.
EDIT: here's some info from the TIS about EWS & keys:
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