Re: Migration
Oracle don't just do databases they do a massive suite of products and have taken over many companies whose products people were relying on. For instance one of the largest hospitality systems groups in the US has been taken over by them and rolled into a division called Oracle Hospitality.
Many products are built around a database and are not ANSI compatible so they specify the database you must use. Oracle have been very good at convincing some application suppliers to use their database exclusively, therefore ensuring a lock in.
If you have a major database backend using specific Oracle DB functions and features then moving can be very difficult. You could be looking at a multi-year project with significant parallel running and downtime, especially if you are trying to use the same front-end and just wish to migrate the DB. You will also need a whole new set of DB Admins and support staff who have all got to work nicely together and understand significant amounts of the limitations of both systems. You'll also be faced with the fact that the 'new' crew are likely to be taking the jobs of the 'old' crew so there could be some ... tension.
If the costs and pain of using DB(x) or Application(y) (Oracle, MSSQL, Applicationx, whatever) are not quite at the level of business critical then you are going to have a hard time convincing the board, the CEO, the users, the IT team and yourself that moving away is the best option.