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Under the rule's impartial conduct standards, brokers and advisors must make recommendations in the best interest of investors, charge only reasonable commissions and fees, and not make misleading statements.

FINRA is not charged with regulating the fiduciary rule. The 2018 priorities letter does not mention the impartial conduct standards.

But the focus on advisory account recommendations appears to be related.

The controversial fiduciary rule, the full implementation of which has been delayed until July of 2019, was designed to eliminate conflicts of interest in product recommendations, and by most evaluations favors the fee-based compensation models common to advisory accounts over commission-based sales of individual investments.

"FINRA will review situations in which registered representatives recommend a switch from a brokerage account to an investment adviser account where that switch clearly disadvantages the customer, such as where the registered representative recommended that the customer purchase a securities product subject to a front-end sales charge in a brokerage account and then shortly thereafter recommended that account be transferred to a fee-based account," wrote Robert Cook, FINRA's CEO and president, in the 2018 priorities letter.

Throughout the more than five years that went into crafting the fiduciary rule, industry opponents of the regulation argued that its preference for fee-based accounts would result in some buy-and-hold investors paying more for access to investment products over an investment lifecycle.

Advisor accounts charge an annual fee based on assets. Lower value accounts are commonly charged 1.5 percent, or more, in annual fees.

Over ten years, a buy-and-hold investor with $100,000 in assets could pay $15,000 in fees, more than the commissions paid in a brokerage account for some lifecycle mutual funds.

FINRA oversees 634,708 brokers, some of whom are also dually registered as fiduciary advisors.

Last October, the Consumer Federation of America, a proponent of the fiduciary rule, sent a letter to FINRA's Cook, the Commissioner of the Securities and Exchange Commission, and the Secretary of Labor, alleging some firms and brokers might be using the impartial conduct standards to move investors into fee-based accounts when it is not in their best interest. That would be a violation of the rule's best interest standard, claims the CFA.

"We cannot dismiss out of hand the possibility that some firms are using the rule as an excuse to shift customers into fee accounts, even when that is not the best option for the investor, or charging them unreasonable fees as a result," CFA's letter says.

"The Department's rule includes provisions specifically designed to protect against this sort of misconduct," CFA's letter adds.

Evidence began to emerge of migration to fee-based accounts after implementation of the impartial conduct standards.

A Deloitte & Touche study of 21 brokerage firms, commissioned by the Securities Industry and Financial Markets Association, showed that 10.2 million accounts with $900 billion in assets had been moved from brokerage accounts to fee-based advisory accounts after the impartial conduct standards were implemented.

And a Fidelity comment letter to the SEC, which is moving to promulgate an alternative to Labor's fiduciary rule, also claimed evidence of a migration from brokerage accounts to fee-based accounts.

"Many investors are now required to pay an asset-based fee to receive exactly the same services that were previously provided to them for no additional fee under a transaction-based fee structure," Fidelity's letter said.

"The rule has thus made it harder and more expensive for retail investors, particularly investors with low balance accounts or investors who simply want to use their broker-dealer to execute a limited number of trades per year, to get the advice they need for their long-term savings goals," the letter added.

FINRA's move to oversee the migration of brokerage accounts comes after the Labor Department implemented a non-enforcement policy for the impartial conduct standards, so long as providers are making a good faith effort to comply.

The agency also said it will also step up its oversight of recommendations to employer-sponsored retirement plans in 2018.

"FINRA will focus on the suitability of firms' and registered representatives' recommendations made to plan participants, including Individual Retirement Account rollover recommendations involving securities transactions. FINRA will also review the supervisory mechanisms firms establish for these recommendations," wrote Cook in the exam priorities letter.