There is facility to correct the detail submitted in GSTR 1 i.e. sales data. Your supplier would have filed the said return. Or you may confirm with him. Since the due date is extended to 10th of October 2017 he may not have submitted. If not submitted then you can ask him to mention your correct latest number. Upon mentioning your correct number, the detail and credit will auto populate in your GSTR 2. The due date of submitting the said return is 31st October 2017. Hence you may procedurally take corrective action.
The purpose of giving practical solution is that GSTN is under going changes/improvement. Department o oofficials are also not in a position to support tax payers. The customer care helpdesk is also able to resolve certain problems. In the midst of that if you ask them how the credit in the shown in old number be reflected or passed on to your new number, then in my view it will take your considerable and allot you stress till the issue is resolved.
Hence, the practical or procedural remedy is the good solution, i believe. Thanks.