Short answer: Nobody knows. Between 0% and 100% of them.
These referendums were conducted as secret ballots, so nobody knows how anyone voted.
Based on that uncertainty, one can only give a minimal/maximal range as an answer.
On 5th June 1975 17,378,581 people (67.23% of total votes) voted Yes for entry into the then European Community.
On 23rd June 2016 17,410,742 people (51.89% of total votes) voted Yes for leaving the European Union.
Now we do not know how old the Yes-Voters from 1975 were because of the secret casting of ballots, but some of them will have already died. Let's roughly assume their age was evenly spread between 20 and 60, so they were born between 1915 and 1955. Let's further assume they get 80 years old, so only those born after 1936 will still be alive in 2016. This means that roughly 8.2 million people of these might still have been alive in 2016.
Some of them didn't vote. Voting age population turnout was 65.38% in 2016. So maybe 5.4 million people who voted Yes in 1975 may have voted also in 2016.
These 5.4 million could have all voted for Yes in 2016 to leave the EU, they also could have all voted for No in 2016 to not leave the EU. Everything is possible. In particular, because voting is secret, you cannot say anything about likely behavior. Maybe indeed they were overwhelmingly still convinced of the EU or maybe not. This information is not available and never will be and it is probably futile to speculate about it.
By looking at results in voting areas you might get to some better bounds but you would add new uncertainty because you don't know who moved to where in between. Exit polls might also give some insights even though they are far from perfect and might suffer from their own error margin or bias.