As a general practice, companies do pay a stipend to a trainee during their training period. The question of salary and basic salary does not arise during the training period.
Of course, the terminology of stipend has also changed, and now it is termed as 'training allowance'.
As a general practice, once the basic salary is fixed, it cannot be reduced afterwards. You can change the basic salary of a person by effecting a hike, but not the other way.