As someone who has done several conversions, one thing I can add to the conversation is that any differences in trail resulting from a switch from 700c to 650b wheels are going to be minimal, and even if the change were to be quite large, the difference in ride dynamics is often unpredictable. For example, much of the way a wide, 650B tire affects steering response is dictated by other aspects of the frame geometry and fit. One of these parameters is reach, which affects how much rider weight is placed over the front wheel. Reach also affects steering because of the lever effect of the stem. A longer stem will have more of a lever effect.

To illustrate-- I did one 650B conversion of a road bike made for 27" wheels. My geometric trail dropped from 52mm to 44mm, and I expected the steering to lighten up and for the bike to handle a front load favorably. I found that this was not the case; on the contrary, steering became more sluggish (which I attribute to the 650B tires being fatter and my having a lot of weight over the front wheel-- my back angle is very low on this bike, around 35 degrees when my hands are on the hoods).

In contrast, I converted a road bike from 700c to 650b for my wife, and in the process built it up as an upright townie. As a result, her back angle is much higher and thus places less rider weight over the front wheel than the frame was designed for. I don't know what the final trail figure is, but I know it's higher than my own 650B conversion mentioned above, probably in the 50s. She complained that the steering was too light. This, despite having higher trail than my bike! To remedy the issue, I installed a handlebar basket and encouraged her to keep some weight in it. So she always keeps her purse and accessories in the basket, and that balances out the weight distribution.

Both her bike and mine have the same tires (650x42B Grand Bois Hetres).

I will soon be taking delivery of a purpose-built 650B road bike frame which will replace my 650B conversion, so I will experiment with reconfiguring that conversion into more of an upright city bike to see if that improves the steering response.

So the take-home message is that you can make all the geometric predictions you want, but geometry parameters other than trail are going to factor in to the ride and handling dynamics of switching from narrow 700c wheels to wide 650b wheels. The best thing to do is to try to get ahold of a set of 650B wheels to test-fit before executing a full-blown conversion. A short ride around the block should reveal a lot about how the handling will be affected.