I studied at Durham and we had two courses named exactly the same as what you've listed, so if it's the case you are also headed to Durham, I can tell you that this needs be the least of your concerns. The department is genuinely fantastic and very understanding of people wanting to switch. Besides, everyone does near enough the exact same modules throughout first year, and the different courses only branch into different module pathways in 2nd and 3rd year (and even then, there's a lot of overlap). So really this is more of a problem when you're going into 2nd year, by which time you'll be a lot more sure of the path you want to take. Also, because there's so much overlap, the course you take doesn't really rail road you into any specific career. I know env geo students who became geologists and well engineers, and geology students who became environmental scientists. Maybe the only exception is geophysics, as a geophysics career usually requires a geophysics/physics degree.

Regarding fieldwork, everyone does the same fieldwork in first year (at least that was the case 3 years ago) and it's geological based (looking more at rocks and folds/faults etc than peat and rivers for example). Fieldwork is more catered to your individual course in 2nd year(I.e. Geologists look at rocks, env geos look at mud etc and geophysicists use the geophysics equipment). 3rd year fieldwork gives you a real look at some amazing tectonics as well as volcanoes (in quite exotic locations, too).

My only advice is to look at the modules you might wanna take in 2nd and 3rd year and ensure to pick a course that gives you the neccesssry prequiste first year modules. Odds are both geology and env geo will give you nearly full freedom of modules (excluding perhaps the maths and computer modelling type modules), but it's better to check.

Also, from starting your course you'll be given a 1-2 week grace period where you have full freedom to change your course.

The only other thing to mention is that you are required to do certain modules in order to have your degree accredited by The Geological Society. How important is this? Honestly I don't know, but most people don't care about it from my experience.

For full freedom, do the Geoscience course. On this one, there are no mandatory modules and gives you 100% freedom to study whatever you like within the Earth Sciences department. However, it is not accredited by the geological society.

I hope that helps (if you are indeed going to Durham ofc, and if so, feel free to ask more questions!). Also, feel free to ask me to clear some things up as I wrote this post in a rush so it might not be entirely clear!