Hi,
You should have received a letter to confirm Tax Credits has stopped the Direct Debit for the debt. You can see guidance here:
https://www.gov.uk/tax-credits-overpayments/universal-credit
When you transfer to Universal Credit any debt is sent to DWP also. DWP should contact you to start recovery of the outstanding balance. We do not have a timescale for this. However, if you are worried you can contact the DWP debt management team. More information on the outstanding balance and contacting DWP Debt Management can be found here:
Repay and manage benefit money you owe
Thank you.