A National focus would probably work to set up the conditions where the events are "possible". After that, best is probably a reoccurring event that checks to see if (for example) you have 30 Pzr III D's in your stockpile and then converts them (with a delay) into (for example) StuG IIIB's.
Probably would have to set up with a trigger something like Earliest Date, # in stockpile, and I think no longer producing the Pzr IIID's but at least one factory producing StuG III B's, and reoccurring. Maybe check once a month to see if conditions are met, or maybe % chance per day check if conditions are met (latter is probably better, as it is less predictable)
AND.....have the option for the player/AI to choose yes/no for conversion to occur.
The key thing is that it should not be free. Some kind of cost must be associated with the conversion, maybe something like 1/10th the cost of a brand new StugIII for a conversion. (Just a guess for discussion purposes).
Would have to set up a separate event for each specific conversion (for example one for PzrIIID >>Stug IIIB, one for Pzr IIIE >>Stug IIIB, one for Pzr IIID >> Stug IIIG etc etc.