Even if your relative did not change denominations, consider the possibility that they attended several different churches. A relative’s second wife attended three different Lutheran churches in the city of Chicago over a fifteen year period. The records of each church were helpful in locating more information on her as the membership rosters provided different details each time. The churches may have been different branches of Lutheranism at the time but that is no longer the case.
It could be the family moved for a short time and one church was closer. It could be that a relative attended one of the other churches for a time. It could also be that she had a falling out with the members of the Ladies’ Aid or the pastor as well.
You never know. The key is to look. Sometimes membership records will indicate former church affiliations or where someone transferred to. Other times they do not.