I live in a district like this and the primary is determined by who is endorsed by the President.
Also these voters are dumb but they aren't that dumb. Unless you know a person who actually has presented as a Trump supporting republican for the last decade and is secretly willing to switch sides after the election, you're not going to trick them.
The point is that center right Republicans (the kind that used to win most Republican districts before 2012) could still win if they could make it to the general election. They often can't because most Republican voters, like most Democrat voters, aren't into party politics enough to bother voting in the primaries.
It is the voters who are most likely to be to be farthest from the center who vote in the primaries, and these are the ones who don't want a normal center right representative.
If Democrats switched parties and voted in the primaries they might be able to counter the usual extreme primary voters so a center right Republican could win.
Also these voters are dumb but they aren't that dumb. Unless you know a person who actually has presented as a Trump supporting republican for the last decade and is secretly willing to switch sides after the election, you're not going to trick them.