Has this ever happened before, that a standing Member of Congress switched parties directly from one major party to the other? If so, when?
The number of party switchers is too numerous to list here. The names and dates are provided in the links.
List of United States Representatives who switched parties, includes the reference to Jeff Van Drew.
List of United States senators who switched parties.
Party switching in the United States, includes a paragraph on Jeff Van Drew under Notable party switchers.
Democrat congressman Jefferson H. Van Drew left the Democratic Party arguing that it was swinging too far toward a radical progressive political agenda that, in his view, did not reflect the will of most people of the United States, and that Trump and what Republican Party had accomplished during the Trump presidency better reflected their will. In commenting on his party switching and reflecting on the shifting political direction of the Democratic Party, Van Drew quoted former President Ronald Reagan as having once said "I didn't leave my party, my party left me." Van Drew, who voted against the Democratic Party line in voting against impeaching Trump, claims that part of his decision to walk away from the Democrats was the alleged behaviour of at least one unnamed Democratic Party powerbroker who - in the days leading up to the House impeachment vote - had allegedly issued political threats towards him, threats that were alleged to be acted upon if he voted against impeachment, with following words to the effect "You will not get the line. You will not get the county. I will do everything to prevent that from happening and everything to destroy you." These words were disputed as "hyperbole" by a person, Mike Suleiman, interviewed by NBC10 who was thought by NBC10 to be the unnamed figure who allegedly issued the alleged political threats to Van Drew.
Motivations, from the above link.
Politicians may switch parties if they believe their views are no longer aligned with those of their current party. Richard Shelby of Alabama left the Democratic Party for the Republican Party, arguing that the former party had shifted more towards liberalism.
A disaffected incumbent who might not hold a leadership position or feels ignored or mistreated by the majority party might join the minority party with the expectation of holding a leadership position in the minority party and if currently elected, having the complete support of the minority party for re-election, who would certainly want to have more elected officials in their ranks.
Some politicians have also switched parties to improve their chances for reelection. Arlen Specter, a former US Senator of Pennsylvania, cited his uncertainty of winning a Republican primary as one reason for his move to the Democratic Party.