changing from paid ticket to reward ticket

if i book a paid ticket and find a reward seat within 24 hours
im i able to canel and rebook the reward ticket without incurring any charges

You can phone VS and 'hold' a ticket for 24 hours after which you (a) phone then up and pay for it (b) let it lapse or (c) phone them and see if they will give you another 24 hrs but VS wont keep doing this for you.

This could be your best option as it means you have a 'held' booking whilst you wait to see if there is a reward seat comes available and no rick of loosing any £

[honey lamb]

It will depend on whether you have booked a flexible fare. If not, I doubt if you will have a chance to change it to a reward fare but in any case there will be charges to make the changes