Here’s an article from the Tiller Help Center How Can I Remove Account Data for an Unlinked or Closed Account. In your case, you would skip the step for removing balances from Balance History.
However, I found the cleanest solution when I want to retain the balance history but don’t care that it started at institution X and moved to Y is to make the Balance History sheet look like it was always with the new institution. I’ve used this method with credit cards and bank accounts, but not investment accounts. Basically, you filter Balance History to the two accounts and then copy/replace the new account information over the old.
- On Balance History, filter by the old and new account
- Copy the values of the new account to the rows of the old account for the following columns: Account, Account #, Account ID, Institution. (if using the drag method, be sure Account # and Account ID are not incrementing.)
On the Accounts sheet, you’ll see the old account now has an invalid flag, because it looks like it never existed. You can delete the contents of the 4 green cells associated with the old account.
The Net Worth and Balances sheet only shows the new account. The Transactions sheet will still show the old institution and account on historic transactions, which doesn’t cause a problem. Here’s a demo file Transfer Account - Retain History.