This is bad news for not just Trump but the Republican party. It shows that people aren't just leaving the the Republican party but actually switching party affiliation all together. Trump will have no choice but to try and steal the election as his adviser Roger Stone has suggested.
The only reason to register in a party is to have a vote in their primary. I have switched a couple times for this. This year I was democrat. It is foolish to think that I will necessarily vote for their candidate in November.
This is only the beginning of the trend, in my opinion. Trump has destroyed the Republican Party. Rather than the "big tent party" of Ronald Reagan, the Republican Party of today is the "tiny tent party", where you can agree 100%, or you aren't welcome. It will be at least a dozen years before it can regroup, if ever. I tend to think that the Trump wing of the party will continue to be dominant, and that the party has much further to fall.