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You can use .reverse:
sage: x = PowerSeriesRing(QQ, 'x').gen()
sage: y = exp(x) - 1
sage: y.reverse()
x - 1/2*x^2 + 1/3*x^3 - 1/4*x^4 + 1/5*x^5 - 1/6*x^6 + 1/7*x^7 - 1/8*x^8 + 1/9*x^9 - 1/10*x^10 + 1/11*x^11 - 1/12*x^12 + 1/13*x^13 - 1/14*x^14 + 1/15*x^15 - 1/16*x^16 + 1/17*x^17 - 1/18*x^18 + 1/19*x^19 + O(x^20)
