Segment clients before you move anyone
Not every client is equally easy to move. Sort your list by complexity: number of bank accounts, transaction volume, whether they invoice through the software, and how current their books are. Simple, up-to-date clients make an ideal first wave. Complex or behind-schedule clients should wait until your team has the process rehearsed.
Also sort by relationship. Clients who trust you and respond quickly will give useful feedback. Clients who are anxious about change need more hand-holding and should move once your instructions are polished. A spreadsheet with both dimensions will tell you who goes first.