No, you do not ask the custodian to change the Form 1099-R, they prepared it correctly. You must split it yourself for entry into TurboTax. If the amount rolled over and the amount subsequently converted are the same, your two split Forms 1099-R will simply have half of the originally reported dollar amount in each of the boxes 1 and 2a on the forms that you enter into TurboTax in place of the original.
