Assuming you haven't committed the object yet when it's new, you could do the following in the button caption:
1. Edit your caption
2. Set the caption to {1}, which is a parameter we will fill underneath, then click "new"
In the edit template parameter window, choose "expression"
In the expression, type something like:
if isNew($Trainee)
then 'Save'
else 'Update'
Now save your parameters, and that should do the trick :)