Can I use pt-online-schema-change to change a primary key?

mysql,percona,pt-online-schema-change

Well, don't do that. Running pt-online-schema-change on a table w/o a single column unique index, may result in a loss of data. See #3 here: http://www.pythian.com/blog/important-caveats-when-using-pt-online-schema-change/