"All revenue was earned through 1099's."

But that's the issue, isn't it?

Whether there is a 1099 issued does not matter. What matters is that the business reports its revenue. Revenue is not only based on 1099s that are issued, but based on Sales. 1099-NECs issued for S Corp labor are not what to rely on for S Corp Revenue, because when there is no requirement to even issue any 1099, what's to prevent you from deciding there is not even a reason to file, since you see no 1099?

An S Corp doesn't even "earn" through 1099-NEC for labor. Customers sending one is not required and is moot and perhaps the W9 has been overlooked. It doesn't matter how many are sent. I took it not literally but as a relationship explanation, that the taxpayer under their Sole Proprietorship and now their S Corp is not an employee of any customer and that was stated to explain this taxpayer works independently from the customer as in the past, but under this new entity type.

Specifically, if you only have 1099, that doesn't mean you have only that amount revenue to report. If you have no 1099, there still likely is revenue to report. If you have this one "missed" 1099, that does not by definition become a missed $750 of revenue, because all of the revenue was supposed to be reported, even if there is not a 1099 to match to every revenue $ that gets reported.

If the business can't reliably track and manage their revenue without waiting for 1099s, that is a separate problem that should be addressed, because as far as the IRS is concerned, an S Corp only gets reported via 1099 in limited circumstances.

What should have happened at the formation of the S Corp includes notification to clients for new contract terms since the shareholder-employee now contracts to customers as President of the Inc, not personally, and the S Corp W9 is submitted as for reference only and "not needing" a 1099, giving out the FEIN of the S Corp.

TL;DR: The 1099 is meaningless and out of context with total revenue. If total revenue included all revenue, then this $750 was already reported. There is no need to amend.