Given that I'm not sure even 50% of Americans have $5k in income tax due in a given year, seems like it would have to be a real refundable credit to make any sense at all.
The devil's in the details, as usual. If it's straight refundable, it's not regressive. If it's NOT refundable, then it's an enormous shaft to the middle class, who don't generally have enough tax liability that it would help them.
The sad thing is that the details should have been worked out and made public for a proposal like this in, oh, say 2008 or 2009. Or at the very least, today. The throwing-shit-at-the-wall-and-seeing-what-sticks process is important, but it's a little late to be doing that when you're already canceling the funding for the existing system.