I'm not so sure about that actually because it wasn't really perceptible in the way he conducted his races until much later. I'm thinking of the 1985 Portuguese GP for example, a race best remembered for being the stage is Senna's first win in the Lotus in apalling weather conditions.
On this occasion far from being too conservative Prost was actually criticized (by James Hunt among others) for taking too many risks in his battle with Elio de Angelis for second. For whatever reason straightline speed was McLaren's achille's heel that year, and it was instantly clear from the word go that the Tag/Porsche had been left behind in terms of top-end power.
Prost at that race was getting increasingly frustrated at being stuck behind Elio while Senna was romping away with it. Many times he got alongside on the start/finish line but by the time they approached the end of the straight the Lotus would gain several car's lengths on him. You could see that this was going to end in tears...
Drivers don't normally get critised for going off aqua-planning (not a lot the driver can do about it) but in this case Prost did because the argument was that he was following him so close and relentlessy behind that ("how can he see anything?"... - James Hunt) he' d stand no chance of spotting any potential rivers forming across the track until he actually drove over it. Which is exactly what happened.
So I think whatever changes took place in his driving approach took place at the tail-end of 1985, and the influence of Niki Lauda was a more inluential turning point for him, especially after his immense disappointment of the previous year and being beaten by him by half a point despite almost always being in front of him during races.