Have the rules for 401(k) in-plan Roth conversions changed?
Yes. Thanks to the American Taxpayer Relief Act of 2012 (ATRA), the rules for making 401(k) in-plan Roth conversions have gotten substantially easier. (These rules also apply to 403(b) and 457(b) plans.)
A 401(k) in-plan Roth conversion (also called an "in-plan Roth rollover") allows you to transfer the non-Roth portion of your 401(k) account into a designated Roth account within the same plan. The amount you convert is subject to federal income tax in the year of the conversion (except for any nontaxable basis you have in the amount transferred), but qualified distributions from the Roth account are entirely income tax free. The 10% early distribution penalty doesn't apply to amounts you convert (but that penalty tax may be reclaimed by the IRS if you take a nonqualified distribution from your Roth account within five years of the conversion).
While in-plan conversions have been around since 2010, they haven't been widely used, because they were available only if you were otherwise entitled to a distribution from your plan--for example, upon terminating employment, turning 59½, becoming disabled, or in other limited circumstances. But in that case, you already had the option of rolling your funds over (converting) into a Roth IRA.
ATRA eliminated the requirement that you be eligible for a distribution from the plan in order to make an in-plan conversion. Now, if your plan permits, you can convert any vested part of your 401(k) plan account into a designated Roth account regardless of whether you're otherwise eligible for a plan distribution. The IRS has also just recently issued regulations that provide additional clarity on how in-plan conversions work.
Caution: Whether a Roth conversion makes sense financially depends on a number of factors, including your current and anticipated future tax rates, the availability of funds with which to pay the current tax bill, and when you plan to begin receiving distributions from the plan. Also, you should consider that the additional income from a conversion may impact tax credits, deductions, and phaseouts; marginal tax rates; alternative minimum tax liability; and eligibility for college financial aid.