A new Opportunity with the same Products can be created from an existing Subscription that might be expiring by using the Renewal feature.

Details from the Subscription are copied over.

No additional product items are allowed for a Renewal of Subscription.

Open a Subscription record.
Click Renew.
A new Opportunity record opens, with defaults:
Type = Renewal
Sales stage = Negotiation / Review
Term (months) = 12
Start date = End date of Subscription + 1
End date = 1 year from Start date
On Subscription Details, set a new Term (if needed) by changing the Start date and End date as prefered. Term is automatically calculated.
On the Products section, enter additional Quantity, Unit Price (as needed) on appropriate line item.
Save Opportunity.

Open an Renewal opportunity.
On the Sales stage field, select "Closed-Won"
A prompt displays that a "Subscription record will be updated." Click OK.
Open the won opportunity.
On the Subscription details section, click link to Subscription record.
Associated subscription record opens.
Validate details.
Term, start date and end date are the same as the Renewal opportunity.
On the Products section, Quantity , Discount applied, and tax if applied are the same value as Opportunity.
Subtotals and Totals computation is the same.
Subscription value field on the header is calculated as the Quantity * Unit Price * Fullterm value - Discounts.

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