Since as far as I know #3 bar is the smallest normally used in construction, if the rebar in the beams is thicker than #3, it is thick enough to be as part of a concrete encased electrode.
To answer your other question, here at least the code version in effect at the time of permitting applies to the project; I'm not aware of any jurisdictions that would apply a newly adopted code to ongoing projects, but perhaps there are some.
Under the 2002 NEC, the requirement on which electrodes to use said to use all "available" electrodes. This led to a debate about the concrete encased electrodes. On the one hand, for new construction, they are available in the sense that with proper planning you could use them. On the other hand, common construction practice was for building sites never to see an electrician until well after the foundation was poured, at which point the electrode was not "available". Certainly I expect that under 2002 NEC, there were not many inspectors requiring the use of the concrete encased electrodes.
The 2005 NEC clarified this by changing "available" to "present". Nowadays there are plenty of jurisdictions where the builder will be required to jack out concrete to access the rebar if they fail to connect to it as a conrete-encased electrode.
Hope this helps.
Cheers, Wayne