I don't think that you really need to get ONLY those who started prior to 1976. If you truly seek those out, you will find they are minimal in size. There were not a ton of people who really were there at the beginning. Because there were not a ton of those, and many of them are older, you may find that there are little to no responses. It wasn't exactly easy peasy for most of the people out there to actually even find out or know about D&D.
I'd probably just ask about those who started with the original Dungeons and Dragons and the original 3 booklets. They will probably have a general experience that relates to the spreading popularity of D&D and the eventual transition.
For example, I WAS a wargamer. Actually, I still AM a War gamer. There is a difference between a War gamer and a Fantasy Roleplaying game. As someone who actually really liked the historical aspect, at times, those who portrayed fantasy into history may have gotten on my nerves at times. Sometimes it still annoys me when people try to insist their fantastical ideas are true to history.
I did NOT own a copy of the rules when I first played (and I imagine there are many who would be in my same spot. They did not own copies of the game at first). I didn't even really know what I was doing or what exactly the rules were, but it fascinated me. I loved it from the beginning. I eventually got my own set of rules.
When Greyhawk entered into the scheme, it changed the game. I prefer to play with the Greyhawk rules. That's my style. Other supplements (and Strategic Review if you were so lucky as to land copies of it or had a friend and...umm...they had a way to transfer information to you somehow) just added to it.
I'd say open it up to anyone who started playing back then and not just those at the start of it all. They could be from 1975 or later, as long as they started with the original 3 booklets prior to the releases of AD&D and such.
Those who were young at the time are now in their SIXTIES or OLDER at this point, if that tells you how old the original bunch of gamers were. Gygax would be 85 now, turning 86 this year. Arneson would be 76. Mentzer is younger at 72. Rob Kuntz, one of the younger ones who started is now around 68.
I know Mr. Kuntz also browses the forums around here at times, but you are trying to talk to an older crowd, most of whom probably do not frequent the forums as much these days.
Just my thoughts on who you should be asking to get a bigger source of information. Still, perhaps the thread will get a ton more responses.
Good Luck!
PS: My impression on how it impacted the existing population...which population? Wargamers were a mixed bunch. Many of the older gamers I FEEL (I have no hard stats or anything for any of this, so it's all based on anecdotal and distant memory) didn't have much interest in this new upstart game. Others snorted with derision. Then you had others who had an interest and then dove deep into it. Varied takes on the Wargamers. On the general population of the world...almost zero impact. It was more localized at the beginning. If you were from Indiana, or the areas that were around Northern Indiana and were among certain groups you probably heard of it, but probably didn't actually have anyone who had it or tried it till later. Even among that population though, unless you were from a certain crowd, it is possible you didn't even hear about it until the late 70s at the earliest. It gained ground among some groups and grew very popular, but was still sort of niche (IMO, remember, this is ALL anecdotal and from memories from LOOOOONG ago). I had relatives from the Indianapolis area and further north that didn't even hear about D&D until the 80s.
I'm not sure what you are looking for when you ask about what impact it had on the population.
