If the project is developed in a clean way you only have to change the
GUI part. This should not be too difficult for most applications unless you have a complicated
GUI which makes a big part of your project.
If you made the mistake and mixed the
GUI part with the business logic behind you will have to rewrite much more code. In this case this mistake will cost a lot of money in form of development time...
For Android you have the choice to use XE5, there is quite a bit information already available, or to use a client/server approach. Only you can decide which approach matches your needs...
As a rough guide:
You mentioned a
MySQL database, so a client/server approach could match, but you would need an online connection to
access the database. If your application could run locally as app without any connectivity a XE5 app should match better.