When did TLG change the small posts on the underside of the modern bricks and plates , eg 1x6 and 1x4, to hollow ones?
I can confirm that some bricks have hollow posts. I ran across this in attempting to answer a "how much does a brick weigh" question. What I thought was a homogenous batch of bricks turned out to have a few with hollow posts. The bricks were all purchased new in the last three years.
From a "material cost productivity" standpoint, it will be in TLG's best interest to migrate to the hollow posts since parts from these molds will require less material to serve the same function (but not immediately). An expedient time to do this would be when a mold is at end of life requires replacement. So, I would expect to see significant variation continue for years to come. The balance point is the cost of replacing a single mold with the reduction in processed material costs over the expected remaining life of the mold. So the older the mold is the less cost effective it is to replace prematurely, but the newer one represents less time in service to amortize the mold over fewer piece parts... both of these things mean that the replacement will likely be when the mold normally reaches end of life. Since the useful life is very long, it is more likely that we are seeing parts from line or factory expansions.
Answering the question tl:dr style: Around the 90's.
Long story, quote from another user:
I recall having read somewhere that LEGO didn't replace all the molds in one go during the 90's. LEGO kept using the old molds until they were worn out. This meant that both solid and hollow tube bricks and plates were present in sets during that time, sometimes even in the same set.
This is an example of the main difference between plates: