It's also worth noting that switching from ANSI to ISO 216 paper would not be a substantial physical undertaking, as the short-side of even-numbered ISO 216 paper (eg A2, A4, A6, etc) is narrower than for ANSI equivalents. And for the odd-numbered sizes, I've seen Tabloid-size printers in America which generously accommodate A3.

For comparison, the standard "Letter" paper size (aka ANSI A) is 8.5 inches by 11 inches. (note: I'm sticking with American units because I hope Americans read this). Whereas the similar A4 paper size is 8.3 inches by 11.7 inches. Unless you have the rare, oddball printer which takes paper long-edge first, this means all domestic and small-business printers could start printing A4 today.

In fact, for businesses with an excess stock of company-labeled #10 envelopes -- a common size of envelope, measuring 4.125 inches by 9.5 inches -- a sheet of A4 folded into thirds will still (just barely) fit. Although this would require precision folding, that's no problem for automated letter mailing systems. Note that the common #9 envelope (3.875 inches by 8.875 inches) used for return envelopes will not fit an A4 sheet folded in thirds. It would be advisable to switch entirely to A series paper and C series envelopes at the same time.

Confusingly, North America has an A-series of envelopes, which bear no relation to the ISO 216 paper series. Fortunately, the overlap is only for the less-common A2, A6, and A7.

TL;DR: bring reams of A4 to the USA and we can use it. And Tabloid-size printers often accept A3.