Changing data type from mediumint to int without downtime
While Rolando has my vote, because he is right in your case, I want to answer your original question (for you in the future and for others) regarding percona toolkit:

pt-table-sync is not what you want. Think about pt-online-schema-change. While there are some hard corners, it works on RDS with the right configuration or minimal changes on the script. pt-table-sync will also fix the issues with rds in the next release. There are other alternatives for online schema changes, oak-toolkit and Facebook ones.

As an alternative, you need a spare slave that can suffer the lag for the time the ALTER is running and then doing a controlled switchover.