Hello Yeepie Racing,
The ability to change from wet tyres to dry tyres, on the grid is dependent on your Stint 1 tyre choice.
If you choose a dry compound for Stint 1, and Advanced Strategy changes this to a wet tyre, you will have the option to change back to your original dry tyre selection in the strategy menu.
If your first tyre selection is wet, you will not have this option.
More information on Advanced Strategy can be found in this thread:
Advanced Strategy in a nutshell.
I hope this helps.