Classes are a tag to follow the money, so to speak. The only reason to change the class of designated funds would be if the trustees reallocated monies. As long as the building fund keeps growing from contributions it does not matter which bank account it resides in. Transfer between bank accounts is simply moving money from one hand to the other.
If you do need to reallocate designated building funds to missions, for example, you can change the class through a journal entry even though you are not physically moving any money. Money on hand is a current asset so a journal entry will use a single account where at least some of the money is. It involves Credit account for existing class and Debit same account, same amount for new class.
You can do the same in a check or deposit with positive amount new class and negative amount old class with net check/deposit being zero