When to deliver training?

This is not an Articulate product question per se, but I will be interested in any suggestions anyone may have.

I'm developing a training program for a software utility. 50 people will need to use it immediately upon the software utility being made available. A larger audience of 300 people may have to use it some time in the future if it becomes relevant to their job (which may be anytime from the day after it's available to a year or two after it's available to never).

An influential manager would like to train all 350 people at once on the theory that 1. it will get it over with and 2. everyone will have been "trained and know how to use it" when/if they have to use it.

This goes counter to the principle that adults acquire new learning best when they have an immediate need for it. Not to mention the unlikelihood of anyone remembering something from a training they took a year or two in the past and haven't thought about since.

But what I can't quite seem to put my finger on is an article, white paper, book, or other reference that I can provide as documentation. Web searching has turned up a ton of chaff, but no wheat.

Anyone in the forum able to point me to a reference?