If your converting the room to a habitable space then you'd have to apply for Building Regs approval to which they would want the stairs changing.
You can get complete staircases from Screwfix for £430 which I think is pretty cheap really.
If you decide to go without applying for Building Regs then you can keep your old stairs but you can't ever use the room as a habitable space for if/when you come to sell the property. You could find that if/when you sell the property the new owner (read solicitor) would insist on the correct paperwork meaning that you'd have to convert at that time to whatever the regs would be on that day (not by todays regs but the regs of the future......!).
There's also fire and ventilation issues that you would have comply with which make the room more useable and (god forbid) safe in case of a fire in the building!
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