The additional appearance puts the new attorney "of record" for the Plaintiff. The original attorney is still "of record" until he or she withdraws or is substituted for. If the intent is to have a true substation of new attorney for old attorney (without the old attorney staying in the case), then I believe a substitution best accomplishes that.
An additional appearance is just that - additional. Unless there is a withdrawal for the original attorney, that attorney remains of record also. The form is basically the same appearance, with the word "additional" or "supplemental" added to the title. There is no added filing fee. If the first attorney withdraws, it is best to either file a substitution of attorneys or secure an order allowing the first attorney to withdraw. That makes for better bookkeeping for the clerk's office.
I agree with attorneys Donselaar and Goldstein.
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