Once we move overseas will will basically have no income in the US - maybe some taxable investment accounts depending on how much our wages increase in the meantime. Can we convert an amount of the standard deduction + personal exemptions ($24250 in 2014 including our child) from our traditional IRA or 457 to a Roth and pay no taxes for the year? We'll be working overseas, but I'm not planning on making enough to go over the foreign earned income exclusion (we'll be downshifting significantly). Is there anything I need to be aware of if we move forward with that plan?
Unfortunately your overseas income still "counts against" the deduction+exemptions because of the way the foreign earned income exclusion works: it's not "tax on (income - excluded income)" but rather "tax on income - tax on excluded income." To put it another way, you are excluding the first $X rather than the last $X. Check out the Foreign Earned Income Tax Worksheet for 1040 line 44 in the form 1040 instructions.
Of course, if your overseas income is less than deduction+exemptions, you should generally be able to convert the difference US tax free (I have no familiarity with the US-UK tax treaty or UK tax).
I am not a tax expert, but you may want to prepare a sample return using your projected numbers to see how it works. Remember you can take the foreign earned income deduction instead of the exclusion if that is better for you (see pub 54).
You will probably find Jeremy's post on his situation helpful, you're basically in the same situation as him. http://www.gocurrycracker.com/go-curry-cracker-2014-taxes/
The difference is that Jeremy has almost no regular earned income, which leaves him plenty of deduction+exemptions room to convert IRAs. In that situation, it's a very effective strategy!