I don't think there has ever been a conversion from other to Nuclear power?
Seems an excessively large/risky (technical risk) change to me.
Think of it like a land vehicle engine arrangement.
They didn't have a nuclear power plant powerful enough for a large cruiser like the Kirov so all the Orlan class cruisers had a combined propulsion system rather than a pure nuke system
In land vehicle terms think of the BTR-60 which had two small petrol engines that were widely used by common trucks of the period.
The problem was that one such engine was not powerful enough for an armoured BTR-60 so they simply put in two engines. The BTR-70 upgraded those engines with more powerful models but it was not until the BTR-80 where a single rather more powerful diesel engine was used greatly simplifying the engine and transmission arrangement of the vehicle.
The Kuznetsov was not nuclear powered because there were no suitable nuclear power plants available to power it.
New NPPs have been developed for the future carriers and have been put on board ice breaking ships already for testing... putting them on board an existing vessel would not be that hard... the Gorshkov/Kiev class carrier had extensive modifications for the requirements of the Indians including the propulsion system completely being replaced with a different system.... it would not be impossible and would be beneficial in terms of space and experience...
It would certainly not be easy, but it would offer a range of clear benefits as the existing propulsion system is reportedly not optimum.
Converting it to an electric boat with propulsion pods rather than a large shaft driven system would allow better use of internal space and the separation of the nuclear propulsion module in a protected area in the centre of the vessel, while the propeller pods could be distributed around the vessels hull making the use of tugs and the vessels manouver capability exceptional.
A NPP would be more compact and without pressurised pipes all over the place and no enormous shafts... and of course freeing up the heavy fuel space needed for sailing around... that could be used for more aviation fuel for the aircraft and make getting anywhere much faster and make logistics easier too.