There are a lot of posts using bash commands selecting the tables from a table in the schema and looping through them.
There is actually a nice command that does this already in Postgresql. Here you have:
REASSIGN OWNED BY <user1> TO <user2>
Reassings all the current database objects owned by user1 to user2.
Here is the documentation of this command:
It is weird that it was a long time ago (Postgres 8.4), and many posts were using Postgresql 9.
Anyways... Enjoy :)