Ask HN: Best way to equalize founder stakes 2.5 years in?

Hey HN - a friend and I started a company 2.5 years ago. We started 60/40 originally, but have since worked our way to a 50/50 (we agreed a while ago this was a better arrangement for motivation etc, and has worked well) and need our share structure to reflect that. Wanted to see if anyone has been in this situation before and how to minimize tax burden

Some relevant details:

- C corp with 10m shares outstanding, 5m common stock issued - 3m to 1 founder, 2m to the other.

- no outsiders are involved in terms of investors, board, etc. no money raises, bootstrap from ground up

- $2.5k rev 2014 -> $370k revenue FY 2016

Gift? private stock sale? Issue shares?

Thanks