We have all been there, working many hours on a recent artwork and you are almost finished, yet you think you need to make a few critical changes and you don't want to ruin it!
One technique I have found a life saver to use when considering those 'what if' moments, is to go into the kitchen cupboard, pull out some baking paper (tracing paper is probably better), and work out those critical decisions first on the baking paper before you actually apply this to your original paper. The transparency allows you to see easily if you made those change what it would look like. This allows for more creative risk taking and encourages you to keep going with your process.
I used this technique with the drawing below and I explored how the face would look if I changed the smile slightly. I'm glad I could take the creative risk first on the baking paper and confidently apply the idea to the original.
Below you can see further progress with the shell. The baking paper also works well to stop your hand from smudging your work.