Yoper,
good to hear from you - here's some answers to your questions below - and I will clarify in the rulebook (and add it to other rough spots you have identified)
"The question here is: Does the early version of an unit that is on the map when the time frame is reached remain on the board? Does it stay on the board in its present form or does it upgrade?
My personal interpretation is that it stays on the board as is."
It remains on the board as is - only the factories are upgraded - for example, the light tanks stay on the board for Italy but Italian factories begin churning out medium tanks in Spring of 42.
"2) Related to that is the question concerning other country charts that only talk about when certain new units are available and don't talk about other units being superseded by these new units.
I take it that is this case, the new units are in addition to the old units?"
Yes
"I would say that if it doesn't specifically talk about these units no longer being able to be built that they still could be built by the player."
Correct
"3) The UK early naval fighter- I take it that the only kind of fighter that can be on a UK carrier before the specific date on the UK country chart is the UK early naval fighter? After that date a normal UK fighter can land on a UK carrier?"
Correct - you can still continue to have your early war fighters on your carriers - but regular fighters can land on Brit carriers after Spring 42.
"Can the UK early naval fighter "land" in a land territory or must it always operate from a carrier?"
Nothing prevents them from landing on land - they can land on either carriers or land.
Hope this helps - good luck tonight!
