You must be a rookie.  You surely don’t have to liquidate.  ACAT

You may have to have the client sign a letter (I prefer to not ask the client to contact the losing firm any other way) to cease active management first, then ACAT.

Client needs to request SB to send a wire to terminate the mgmt agreement, but NOT liquidate. Then when the accounts move over, you reinstate the new contract thru the new firm, assuming the mgrs are on the new firms platform. The model will be the same, so there will be continuity of mgmt, without disruption of the portfolio.