You still only have to pay one lawyer fee - whatever the new attorney's percentage is. The new attorney and the old one then split that one fee proportionately based on the amount of work and time each put in on the case, along with their costs on top of the one fee. Be sure you have another attorney lined up to take the case before you fire the old one because many attorneys won't take a case if another attorney has a right to a portion of the fees.
Your total fees are capped and are NOT increased, no matter how many lawyers you hire, and it's up to the lawyers to split their contingent share. Be advised that sometimes lawyers fight over who's entitled to what, and can litigate between themselves, but as long as you release the agreed on share, that shouldn't concern you.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
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