50 millions rows of data is not really that much data. it sounds like you are being blocked. exclusive schema locks, data , etc.
this is my suggestion what you should do instead:
1. script the table via SSMS with all the constraints and all indexes.
2. modify the script to have a slightly different name, ie CREATE TABLE OriginalName_TMP
3. modify the script to have the new identity field you want.
4. create the table by running the modified script.
5. rename the original table so people can no longer access it(sp_rename 'dbo.OriginalName','OriginalName_SAV')
6. Insert the data into the table with the new structure (INSERT INTO OriginalName_TMP(ColumnList) .... SELECT ColumnList FROM OriginalName_SAV)
7. rename the tmp table to the original name sp_rename 'dbo.OriginalName_TMP','OriginalName'
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