Just to be clear, you're saying that the strike of the option grants (or the 409a) in 2014 was roughly based on a 10b valuation? That does seem to screw over anyone with such options, since it's unlikely they'll see appreciable gain on those. Is this sort of conversion from existing option grants to RSUs common? I know most startups convert to RSUs for new grants eventually, but was unaware that some also apply it retroactively. It's a nice show of good faith to convert them to some # of RSUs on Dropbox's part.

It's not uncommon for a company to take a look at the equity incentives it's given to employees, in whatever form and realize it's not worth very much and try to make good by giving replacement incentives. I've been at companies where they replaced underwater options at $N a share with at money options at $M with $M being much less than $N (of course the stock continued downward and I left for other reasons before anything vested); for a not yet public company, poorly priced options being replaced with RSUs makes a lot of sense, because the company may not be able to increase its valuation, but it doesn't want to decrease it, and options issued under FMV don't qualify for exciting tax programs.