I am a registered Democrat, and I would like to vote in the upcoming Republican primary. I understand you can sign a card the day of the election to change to Republican, then later change back to Democrat. Is this possible?
No, state election laws prohibit changing party affiliation on election day.
The card you mention is for unaffiliated voters, those who did not choose a party when they registered. They, in fact, will be able to choose between voting in the Democratic or Republican primary and, in the process, will align themselves with one party or the other. If these voters wish to remain unaffiliated, they'll need to fill out a form at the county election office after the Nov. 5 general election.
Voters are free to switch party affiliations whenever the voter registration books are open. In the case of today's primary, the books closed July 22. They'll open Wednesday.