Curved bay window sill

Yes this is possible to do. depending on materials would change the difficulty in doing the job. If it were to be done in pine or hardwood finish would be more difficult and time consuming compared to using MDF sheet material.

I depends. If I is a timber window it is impossible to fit a new projection sill without removing the window as there will be through mortice and tendon joints and mortices need to be cut in the new sill then fitted. However it is usual that if the sill is rotten the the ends of the frame will have also rotted. If it is just the internal window board you are referring too then, yes easy to replace. Regards Norman Styles

Are you talking window board ( inside) or cill ( outside )The curve is difficult once the window is in place but possible if you know what you are doing use only hardwood (external not softwood and definitely not MDF unless internal

Yes this in possible to do first i would look at what your other window seals are made of to give you the exact finish to suit the rest in the home. If you are going to use MDF i would use green MDF which is moisture proof for any draft or moisture that may occur beneath as when you take the existing off you will probably notice there is a cavity. If you are using a hardwood you want to make sure this is treated before on the underside before fitting.