Refixturing an apartment,just replacing one for one.Upon starting we discover that existing fixtures are fed with flex but no ground installed,by code does the wiring have to be changed or is an as built condition similiar to a situation of adding a plug to existing knob and tube?

You are going to be required to bring any electrical wiring that you are dealing with up to the current code. You will need to make sure there is a bond in that flex, or run BX as a replacement. As far as the receptacle being installed on knob and tube......it is also required to be bonded. You can get around this by installing a GFI in it's place.

Receptacles: 2006 26-700(7)(8)&(9)

Lights: I've never heard of an electrician telling a customer that they need to rewire a part of their home to install a new light fixture. If we get to that point, they'll just get the neighbour's son to do it. I'd be pretty impressed if you could talk someone into putting a GFI in the circuit.

Already, with the AFCI rule for bedroom plugs, by the time a customer gets a couple quotes, the bedrooms are renamed dens, sitting rooms and tv rooms.

Happens all the time. I understood that if it has a closet it's considered a bedroom. I haven't found anything in the code on this though. Has anyone else?

1-As far as light and a GFI, I am not sure where you are getting this comment from as I was talking about bonding conductors with respect to the light replacement, and GFI with respect to installing receptacles on knob and tube.
2-There was never any mention of a bedroom circuit, so I am not sure how AFCI's were brought into the equation.

As far as bringing existing electrical work up to code, look at rule 2-022. I am pretty sure the inspector is going to want a bond wire for that light, especially if it is constructed of metal. Besides, for a breaker of fuse to work properly should a fault condition exist, you have to have a good path back to ground to operate that fuse or breaker. Also have a look at 10-400(g)

If it's a fluorescent light; it won't work right without a ground anyway.

That is another excellent point!! When I was an apprentice I once seen a journeyman pigtain the nuetral to the florecent light to make it work. Not a good move.

The requirement to upgrade is not well defined in codes. Here are the two main points in changing a light fixture. Wire temp rating required for new fixture and requirement for bonding. If a fixture replacement requires bonding as would for a 2 wire receptacle on K & T wiring or the new fixture requires a higher temp rated wire than is available in the outlet box then an upgrade is required. Even an antique fixture that is rewired will be rewired with a never before required bonding conductor.
If requested I have approved gfci protection where bonding a new fixture is impractical.