My scope calls for outlet replacement, are arc fault outlets required?
If the job is in a jurisdiction that is under the 2008 NEC, no. 2011 NEC, yes. 2014 NEC, maybe. A new 2014 exception has been added to 210.12(B) that is invoked by 406.4(D)(4) and the broad "as specified
elsewhere in this Code." The 210.12(B) Exception relieves the AFCI requirement if the "conductors are extended less than six feet". With this wording, the "extension" of conductors done by adding pigtails at the receptacle outlet can't be excluded from the exception, IMHO.
The existing wiring is all BX with metal boxes. If I install at the first outlet and it doesn't hold, then what? Do I change them all to arc fault at $25.00 each?
If the existing BX branch circuit won't hold an OBC AFCI (with load side connections) at all, and troubleshooting doesn't immediately find an obvious wiring error, I'd be inclined to do the rest of the receptacle replacements on that branch circuit all with the OBC AFCI, rather that replace the BX itself.
A side question, about the BX, what generation of BX is it? Does it have plastic or rubber insulated conductors? Is there evidence of the sheath bonding conductor? Will you use the metal sheath as the EGC?