Sales return in GSTR 1 in case of change in Constitution

If you have not surrendered your earlier GSTIN (proprietorship) then show it in proprietorship and file the returns. Else raise a credit note in this GSTIN, the government is not entitled to keep that tax anyway.

First At the time of Proprietor to LLP 

You have Transfer Closing Stock by filling Form as mentione Mr. Pritesh Sir

In Your case its not a Sale Return Because 

Propritor and LLP are two different Entity 

So My Opinion its Not treated as sale Return