Phillip Long has a thoughtful and helpful post here on whether Paul was converted or commissioned. Count me in the conversion camp. But I do think that part of the issue relates to definitions and one must be very carefully not to import our modern conceptions of conversion into Paul's experience. That being said, one issue for me is that I don't see sufficient similarities between Paul's experience in Acts 9, 22, and 26 and Old Testament commissioning types. Four prominent examples illustrate what I mean: Moses, Isaiah, Jeremiah, and Ezekiel.
Notice that in the case of Moses, Isaiah, Jeremiah, and Ezekiel, their commissioning was not in the context of rebellion against what the Lord was doing as was the case with Paul.
Moses, Isaiah, Jeremiah, and Ezekiel all seem to have their experiences in the context of sacred space. With Moses, the idea is seen in the location Horeb (aka Mt. Sinai) and with the command to Moses, "Take off your sandals, for the place where you are standing is holy ground" (Exod 3:5). Isaiah has his commissioning in the "temple" (Isa 6:1) with interpreters debating as to whether it is the earthly or heavenly one. The location of Jeremiah's calling is unstated but the Lord's touching of Jeremiah's mouth in 1:9 would seem to imply a divine throne room setting. Ezekiel's commissioning might be visionary (Ezek 1:1) but the description of what Ezekiel sees certainly sets the context of his calling in sacred space (Ezek 1:4-28). In contrast, there is no hint of sacred space in Paul's experience.
Also, with Moses, Isaiah, Jeremiah, and Ezekiel, the organ of speech, the mouth, plays an important role (Exod 4:10-11; Isa 6:5-7. Jer 1:6, 9; Ezek 3:1-3). But for Paul it is the organ of sight, his eyes, that plays a prominent role (Acts 9:8-9, 17-18).
One final point, with the commissioning of Moses, Isaiah, and Jeremiah there is a sense of immediacy. That is, the calling happened at a singular moment. In Paul's case there is a three day delay (Acts 9:9).
Other points of dissimilarity could be raised but these seem sufficient to show that if Paul's experience was a commissioning, that it was substantially different from the examples of commissioning that we have in the Old Testament.