On the usage of “turn” in spy stories

One dictionary that has this definition is the Middle English Dictionary, so I think it's safe to say it's a little older then the OED says:

18. (a) To shift allegiance (to or against sb., someone's side, etc.);—also refl.; also, cause (sb.) to shift loyalties; transfer (one's loyalty to sb.); (b) to change spiritual allegiance, undergo a spiritual conversion; convert (to Christ, a faith, etc.);—also refl.; also, cause (sb., a group, etc.) to undergo a spiritual conversion; change the spiritual nature of (a temple); (c) to apostatize; cause (sb.) to apostatize; pervert (a mind, heart, etc.), corrupt; turnen to baddenesse; turnen oute.

The (b) sense dates back to c1175, so it probably lead to the (a) sense, which dates back to c1300.

In particular, the following quote seems to have a similar structure ("turned him"):

Wiþ Fortiger be þai nold
& turned hem al bi on acord
To Vter Pendragoun her lord;
_{Of Arthour & of Merlin, c1330(?a1300)}

As for whether it is synonymous with "corrupt", that probably depends on which side the person is switching to. If they're switching to the enemy, then you could say it's corruption. However, if they're switching to our side, then you might say they finally see the light ;)

It comes as a very early verbing of a noun, in this case, the word 'turncoat'. Turncoat, in turn, came from the 1100's, in the Rotuli Chartarum where two barons changed fealty from William Marshal, 1st Earl of Pembroke to King John. Thus they quite literally changed their coats of arms to something else. Turned the coat.