Question is if German would manage something like that also in factories manufacturin Pz III/ IVs. Continue production of one while starting Hetzer. Increase production of Hetzer and decrease of Pz. As you said. Hetzer was easier to manufacture so I don't see why it shouldn't be possible without vehicle production loss. I am curiouse if bigger issue wouldn't be training of units where PzIII/ IVs would be replaced by Hetzer as Pz III/ IV production would be decreasing.
Also let say somebody come with Hetzer design sometimes in 1942. Wouldn't idea to cut production of Pz III/ IVs in 1943 and to replace them with Hetzer required change in thinking of German higher command including Hitler. Hetzer is more suited for deffense and you start to manufacture them let say in 1943 you basically acknowledge Germany can't take Soviets out of war and needs tongo on deffensive. All this before Stalingrad and Kursk! And I am mentioning Stalingrad because preparations needs to be made right as Hetzer would show up in early 1942, if we want to see desired effect in 1943.
This addresses those points the wrong order.
AFAIK some of the increase in AFV production in Germany and Czechoslovakia from 1943 was the result of new firms being brought in rather than existing factories increasing their output.
If the above statement is correct if the JPz 38(d) and (t) are begun in 1942 and come into production in 1943 I expect that the firms being brought into the programme will simply tool up to build them rather than the Pz III or IV based vehicles they tooled up to build IOTL. Therefore my guess is that they will build as many AFVs from those sources in 1943 as IOTL.
IOTL production of the Panzer III as a tank ended in 1943 anyway. AFAIK all the factories building the Panzer III tank switched over to the Panzer III based StuGs. I think there would have been some loss of production from these factories in 1943 ITTL if they stop building the Panzer III chassis altogether and turn over to the JPz 38(d).
Production of the Panzer IV as a tank would still continue until the end of the war if the JPz 38(d) went into production in 1943. However, I think that the factories already building the Panzer IV would concentrate on the tank version instead of producing the JPz, StuG and Kubelblitz versions too. There should be no need for them because in common with IOTL those roles were to be filled by the JPz 38(d) and derivatives of it.