So much depends on the database structure, and whether or not you're using the features that PostgreSQL supports and MySQL doesn't, like transactions, sub-selects and stored procedures. While MySQL is very fast, it doesn't offer all of the features of PostgreSQL.
Another thing to consider is the how much coding will need to be changed; if you coded it carefully and abstracted the database calls, good for you. If not, I'd think about this very carefully...
Mom always sez, "If ain't broke, don't fix it."