Although no ethical issues leap to mind, there are other factors to consider. A party can employ former counsel to consult on nearly any matter under the sun under the limited facts you've presented. The issue becomes whether that attorney can then serve a purpose as a true "expert." Merely being an attorney does not qualify one as an expert in all fields. If for instance the attorney is a general litigator, he or she is not necessarily qualified to testify as an expert in a matter involving complex legal issues of insurance coverage or duties to defend. When a party designates an expert, the other side has an opportunity to object to the designation, both at the time of designation and again at trial following the party's examination of the witness if the party believes the qualifications of the "expert" do not in fact qualify him or her to testify as an expert in the field in which he or she is designated by the party purporting to rely upon the expert's services. The court has the power, on a party's objection or on its own motion, to refuse to permit the designated person to testify as an expert. That said, anyone can hire his or her prior attorney to consult or provide "expert" advice during the pendency if a case. Whether that attorney will ultimately be permitted to offer his or her opinion as an expert at deposition and ultimately trial is a different matter altogether.
I agree with my Chicago colleague--it doesn't sound like a problem to me.
I am not your attorney and any posts/messages or responses to posts/messages can not and do not establish an attorney-client relationship. You should not rely upon free legal advice, and I disclaim any liability for the outcome if you do. Any opinions offered on matters outside New York State are for general informational purposes only.
Be careful not to use your former lawyer as an expert in a way that constitutes a waiver of your attorney-client privilege from the prior matter. You write that the new matter is unrelated to the earlier case, but when you make a lawyer a witness of any sorts, you risk waiver of privilege on similar subject matters. Also, make sure you do not have any compensation arrangements with the lawyer-witness that could be deemed contingent on the result of the case.