What is the correct process for creating a new plan correcting an old plan?

Summary:

We need to update a sick accrual plan that was set up to accrue at the end of each month, and correct it so it accrues at the beginning of each month. We need to create a new plan effective 11/1/2025 and make sure everyone's balance is automatically transferred from the old plan to their new plan.

Content (please ensure you mask any confidential information):

We've done this several times this year but for whatever reason, we can never get the associate's old balance to automatically transfer to their new plan.

Does anyone have a step-by-step process document that details this procedure? I'm convinced we're missing a step somewhere.