You may want to check with AMEX directly for the definitive answer (in which case I would like to know). I suspect that they would do a one-time transfer if you were cancelling your German card and doing a global transfer to a $ card. However, when I talked with IDC London last year about potentially setting up a secondary AMEX Euro card, they told me I would in effect have two separate MR accounts-- one for the International Dollar Card and a separate MR account for the International Euro card. They mentioned that I could not link an IDC $ and an IDC EURO card to the same MR account and could not pool miles. Since I have both a U.S.-based MR account and an IDC MR account, I ultimately decided that I did not make enough EUR charges to justify splitting my MR miles into yet a third fragmented MR Euro account. In addition, the $ card is currently more advantageous for MR accrual because IDC has the same point accrual rate for both $ and Euros -- i.e. 1 mile = $1 or EUR 1 charged. And the IDC MR program appears to have the same redemption values for both currencies-- i.e. 10,000 Delta SkyMiles = $10,000 or EUR 10,000 in spending.
Thus, spending currently the same level on a dollar card will yield more points.