After changing from a proprietor firm to a Pvt. Ltd. Company, can the Pvt. Ltd. Company raise bills from the old contract?
I'm guessing from your use of "Pvt. Ltd. Company" that this is not a company incorporated under English law.
For a company to take over the contract under English law, you would need an assignment or novation.
See (in the construction context): https://www.out-law.com/en/topics/projects--construction/construction-co...