We ran into the following situation in the attached randomized alter test log:
Two columns 'a' and 'b', 'a' is a primary key column. In the same alter table request,
1. Rename 'a' to 'c'
2. Rename 'b' to 'a'
3. Drop 'a'
In step 3, 'a' is not a primary key column and can be dropped; however, we err and reject the alter saying a primary key column cannot be dropped.