Charles Schwab's proposed acquisition of TD Ameritrade is stirring up worry among financial advisors.

Schwab on Monday announced it would acquire TD in a $26 billion all-stock transaction. Together, the firms will serve 24 million brokerage accounts, accounting for more than $5 trillion in client assets.

Both firms also have massive footprints in the registered investment advisory industry, where they custody investors' assets and execute trades.

They also provide firms with the technology to simplify their workflow and allow advisors to focus on financial planning.

"Together we can deliver the ultimate client experience for retail investors and independent registered investment advisors," said Stephen Boyle, TD Ameritrade's newly announced interim president and CEO, in a statement.

The merger will create a massive custodian in a field that often has little choice for these RIAs.

Clients are likely to keep their financial advisor, even amid a change in custodians, industry experts say.

That's because RIA businesses generally have so-called negative consent provisions in their paperwork when they take on new clients, meaning if the firm changes a service provider, the client has agreed to go along.

"I might change custodians if I think it's the best thing for you, and by signing this you are agreeing that you will go, unless you opt out," said Danny Sarch, founder and owner of Leitner Sarch Consultants.

Nevertheless, a client who is content with the service he or she receives will likely stay put.

"The client is generally loyal to the advisor, as long as the advisor keeps the client front of mind as he makes a decision," Sarch said.

In the RIA world, there are four custodians that make up the lion's share of the market: Fidelity, Pershing, Charles Schwab and TD Ameritrade.

Schwab, Fidelity, TD and Pershing collectively hold 80% of the RIA firms' $4 trillion in advisory assets, according to Cerulli Associates.

TD's specialty has been on the small RIA market, centering to firms with fewer than $100 million in assets under management, Sarch said.

"Pershing ignores that space, but Fidelity, Schwab and TD have traditionally competed," he said.

Advisors have raised concerns about the prospect of less competition.

"What does it mean for costs for advisors like me and for our end clients?" asked Daniel Tobias, a certified financial planner with Passport Wealth Management in Cornelius, North Carolina. His firm handles custody of client assets with TD.

"Or what if the service goes down, we have fewer options and we're that much more captive?" Tobias asked.

RIAs who are just starting out on their own or whose business model isn't centered on gathering assets have also turned to TD for their services.

In 2016, XY Planning Network, a network of fee-only advisors who charge clients a subscription fee rather than a percentage of assets under management, partnered with TD to make its services available to those advisors without requiring them to manage a minimum level of assets under management.

Indeed, 600 of the 1,100 fee-only advisors on XY Planning Network are using TD's services through this agreement and manage less than $10 million, said Alan Moore, co-founder of XY Planning Network.

"The question is, 'Will they continue to support the smaller RIA market?'" Moore asked. "I hope they see the value of working with people who are starting in the business and growing their firms and who may never have $50 million in assets."

"The custodians have to make money, but there are firms that will never meet those minimum asset requirements, and we're losing the only option we have," he said.

More from Personal Finance:
Flying this weekend? How long you'll wait at the airport
Cancer patients are 2.5 times more likely to go bankrupt
Use the dollars in this tax-advantaged account or lose them