That's where sub assemblies come into play. The complete explanation is beyond the scope of this thread but, in short, there is a degree of deviation to which you can design your product and production/assembly line. The JF-17B's that are currently being produced are small batch production aircraft (like the prototypes and the first 8 JF-17A) i.e. they are not being produced on the primary production line and hence also have a very low rate of production. Once Blk 3's production starts the B versions that we will produce will also be according to Blk 3 specifications. If they are not, then something would have gone terribly wrong with our production philosophy.
There is another major point that I did not mention before. When you can readily produce the newer model of anything, would it make sense to still produce the older model in parallel? Defeats the whole purpose of coming up with the new model, destroys efficiency and throws economies of scale out the window.
These fan made models and drawings have had a bad habit of being closer to the truth; J-31, J-20, J-10B. Now, whenever another one comes out people get excited. Not saying that this one is as well at all.
Why?
To be fair though, with missiles in hand, using aircraft as nuke delivery platforms is pretty obsolete.
Right on the head.