I am in the same situation, I read this, I think it means upgrading but it didn't state rod useage or upgrading for salmon or sea fishing,
How to Upgrade a Rod Licence 2021
You must use the correct type of Environment Agency Rod Licence when you are angling for fish. You must not fish for sea trout or salmon with a Trout, Coarse Fish and Eel licence.
Note: You can upgrade your fishing licence by paying the difference in rod licence cost at a Post Office branch.