Thank you for your question.
Business schools ask for very specific questions; therefore, all-purpose letters written in advance from your current supervisors probably will come across as somewhat generic and won't address each school's specific questions. In addition, in terms of process, the schools generally send a link to your references via email for them to complete the recommendation online, rather than hard copies being mailed.
Instead of getting all-purpose letters, why don't you meet with your supervisors before you leave and ask if they would be references for you in the future, and then make sure to stay in touch with them periodically so that when you are ready to apply, they'll be up to date on your work experience and goals? That will enable them to answer each school's questions with focus and relevance.
Sincerely,
Jessica Shklar
MBA Mission
646-485-8844
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