Obi
Much of what was available prior to 1982 was B Models. B1 & 2 were compatible with each other, but not interchangeable with B3-8. They were very mature and reliable, and most never saw the factory again until returned for the BF Mk2 upgrade in the late 80s. I think only B7 & 8 were upgraded, but retained as the Sample and Reference, and not returned to service. Only enough mod sets had been bought for the (55?) production sets that remained after attrition. I think you are correct to say "further development" is a better term. The original version you speak of had a different Signal Processor and Receiver, and those built at that time were upgraded in about 82/3, and then all upgraded again for the BF Mk2 package. The LRUs that were visible to pilots remained the same throughout, except that the Display chemical filters were replaced with Hoya (?) glass filters. That last upgrade took a long time, partly if I remember because there was a reluctance to return kit that worked well. In the end, there was pressure to complete the BF Mk2 mid-life upgrade before Vixen entered service. There was also a problem whereby RAF suppliers made a 15 year spares buy for BF, just as she was being actively withdrawn at the rate of 3 sets a month during FRS2 conversion. As the LTC line was "SHAR radar" (whatever that may be - Fox or Vixen), this meant there was little left for Vixen, which was poorly supported to begin with. There was queue of potential buyers for the 55 sets, but no-one else was allowed it on security grounds; and if they still existed, that would still be the case even today. (India only had the original version).
I look at it from the "Continuing Design Services" viewpoint. Each of the radars I mention had a defined CDS package, with development to be completed post-Falklands. As you say, some production lines were mostly complete, others were ramped up in early 82, and others had development stopped quite early in the process. I was generalising a little. Sea Spray configuration control was quite difficult - like BF, it had multiple, incompatible variants of the same LRU. For example, you couldn't fit a T2 or T3 Transmitter, with a C1 Control Indicator, or very serious damage would occur. In other combinations, you could not expect Sea Skua to work. Of all these, I think Sea Searcher suffered most, as the CDS funding was chopped around 86 and right up until out of service date the actual build standard bore no relationship to the spec in key areas. Long time ago of course, and memory fades!