No, but as a matter of discretion, this may depend upon how far the appellate process has gone. If you have just received a copy of the transcript, then you have a limited amount of time to appeal. Such a motion to substitute can prove unwise at that point. You may have to file for a continuance for good cause depending upon how comfortable you are with the process.
If the transcript has yet to arrive, then such a motion to substitute can help insure that you will receive it rather than the previous attorney of record. However, you will also have to file the EOIR-27, as well. You may want to check with the BIA Clerk to see if you are recognized as the attorney of record after first filing your EOIR-27.
A BIA Appeal has formalities. There is a DOJ publication that goes over the recommendations and requirements at the BIA website. You will need to decide whether the arguments made by the attorney justify an appeal based upon the record in Immigration Court.
Perhaps, there are justifiable reasons that led to the conflict between the last attorney and your new client. Perhaps, a reasonable fee dispute? A BIA appeal can take nearly as much time to complete as an appeal to the Federal Circuit Court of Appeals depending upon the issues. If you are unaware of the case law, the briefing formalities, and the record of proceedings, then you need to think about whether your entry into the case can prove counterproductive for both you and the client.
At times, a newer third attorney may allege that the previous attorney demonstrated a lack of due diligence. The third attorney may seek to report them to a disciplinary commission based upon Matters of Lozada and Compean. I, for one, would not want to be in that position, even if the claim proves to be questionable at best. Use care when deciding whether you have a worthy client to represent.
The above is general information and does not create an attorney client relationship.