The manual does not address third basemen playing second base. I called Mike about the time of my use of Koskie at second base, and he did not know what effect that would have on his fielding. My anecdotal and small sample evidence, further skewed by my recollection, no paper evidence is as follows:

I played Cory Koskie at second base a few years ago. As you know, Koskie is a good defensive thirdbaseman, who was not rated at any other position.

He played second for about 1/3 of a year. He made numerous errors, but the lack of ability to make double plays was most concerning. He was rated low on the DP rating at third base in spite of his otherwise good fielding ratings.

If a 3rd baseman who is rated high on the DP rating played second for an appreciable length of time, other than making a lot of 2B errors, I don't know what the effect would be.