X'Y'Z' with 2.6 gamma, in fact.
That is correct. So for that reason I would not suggest you simply "flip the switch" on the ODT and output a DCDM. You need to, at a minimum, view that in a screening theatre or grading suite with DCI calibrated projection, and trim the grade if necessary. The new ODT will give you a good start point, and the majority of the colours will look perceptually the same as your Rec.709 grade. But it gives no guarantees about the handling of colours which were previously invisible to you.
In addition, viewing a film projected in a dark room on a large screen may cause you to reconsider grading decisions that were made on a Rec.709 monitor. So you need to view it projected for that reason as well.
There are in fact two DCDM ODTs in the ACES repository – DCDM and DCDM_P3D60. The latter clips to the P3 gamut. The former is unclipped, and any gamut clipping is left to you.
An additional consideration is that of white point. The DCDM_P3D60, as the name implies, is intended for an "assumed observer adapted white" of D60. This means that it maps R=G=B in ACES to D60 white in X'Y'Z'. But if you were viewing Rec.709, with its D65 white, when grading, then you probably don't want a D60 DCDM, unless you graded using the D60 sim version of the Rec.709 ODT so your eye/bran combination was adapted to D60 when you were grading. Some ACES implementations include alternate ODTs for different "creative white points" such as D65. This is because DCI does not mandate a white point, and allows you to master with any white between D55 and D65.