2008 Democratic Primary Exit Poll:
Democrat 62% Republican 9% Independent 28%
http://www.cnn.com/ELECTION/2008/primaries/results/epolls/#WIDEM
2016 Democratic Primary Exit Poll:
Democrat 71% Republican 2% Independent 27%
http://www.cnn.com/election/primaries/polls/wi/Dem
2008 Republican Primary Exit Poll:
Democrat 5% Republican 73% Independent 23%
Same source
2016 Republican Primary Exit Poll:
Democrat 6% Republican 65% Independent 28%
Same source
Your analysis was flawed. It is quite obvious what occurred. Republicans crossed over in 2008 and stayed in the Republican primary in 2016.
That's where the action is at, not "exit polls" or "opinion polls".