Once all 10 positions in the CAS are used, you do NOT need to replace the CAS!.. you simply do an "All keys lost" with any number of tools (CDGI, VVDI, Autel, BMW Explorer, BMW Multitool, or any Chinese clone of the above, etc). The rest of what Gee_115 said is accurate. Your CAS comes with 10 key slots that have been coded with individual security codes. BMW has these codes in their database for your VIN so they can write a new key specific to the next key slot in sequence without the car physically being present.

However, if your car had previously had an 'independent locksmith' program a key in the past they certainly did not inform BMW and therefore BMW could write you a key for slot #3 when, in fact, that key slot/code was already consumed by a previous key. Additionally, you can only have two active "comfort access" keys in the CAS at any one time. If one of your previous CA keys was lost and NOT deleted/marked as 'lost' in the CAS, your new CA key will only work as a non-CA key until someone/you (even with ISTA-P) delete the entry/mark it as lost for the lost key in the CAS.

 

I don't think any issue will be arisen if an independent locksmith can program the key at all.