If I start off as a trainee and want to become an apprentice, is this a possibility?
Yes. It is possible to start on a traineeship placement and progress to an apprenticeship. There are two routes for progression:
1) at the end of your traineeship your host farm decides to offer you an apprenticeship
2) you complete your traineeship and later secure a longer term placement with another approved host
In both cases you will need to top up your fees in order to continue as an apprentice.