I have a level eight moon druid and would like to maybe change to another type of druid. Can a swap Druid circles ?
No, you didn't break the game
The simplest answer here is that no, creating a land circle does not automatically imbalance your game. The biggest factor here is, how versatile and effective are your other party members? If your paladin and fighter are also focusing heavily on optimizing themselves and being as effective as possible, then you shouldn't see any issues. Your DM can just put bigger harder challenges in front of you.
Sustained Imbalance is Bad
The potential problem comes if your circle imbalances the party. If this makes you twice as effective as the rest of your party at every level, then you're probably going to reduce the amount of fun had at your table. Your party mates and DM may become frustrated by how ineffective they seem.
Talk About It
The best advice I can give you is to talk about it with the other party members and your DM. I would also encourage you to be open to the idea of making changes after you see your abilities in play. You mentioned being fairly new to D&D so I'm going to assume most of your group is as well. This means you don't have a giant frame of reference. IF after a dozen sessions you seem vastly more powerful than your other party members, be ready and willing to adjust your circle spells to something a bit less powerful.
You have it all basically correct
So I can prepare wisdom mod (lets say +3) + level per "instance" (before doing a long rest). (So I can prepare 6 spells)
Yes, you would have 6 spells prepared each day. However, you get your spell slots back and you get to re-prepare spells after a long rest, not before. You would need to complete the entire long rest before you prepared new spells and got your used spell slots back.
I can choose any 6 spells out of the level 1 and 2 list (since I only have level 1 and 2 spell slots)
Yes, there are no restrictions on which spells you can prepare, so you can prepare all level 1 spells, all level 2 spells (although that isn't a very good idea with only 2 level 2 slots), or any combination of the above.
I can cast the same spell as MANY times as I want (like I can cast goodberry (a lvl 1 spell) technically 6 times (4 level 1 spell slots, and 2 level 2 spell slots). The level 2 slots will increase the damage of course.
Mostly correct. Not every spell increases in potency when cast at a higher level. In fact, the Goodberry spell you mentioned does not get any better when cast using a level 2 spell slot. Many do, but some do not. Keep that in mind and make sure to read the spell description before you use it in a higher level. To be clear, you absolutely CAN cast a low level spell in a higher level slot, however if there is no additional benefit for casting the spell in a higher level slot (as per the spell description) then you may not be optimally using your limited spell slot resources.
but I could only cast spike growth (a level 2 spell) 2 times, because I cannot cast it as a level 1 spell.
I could do a long rest and re-prepare 6 more spells and my spell slots would all be refreshed.
Yes, with one minor caveat. Keep in mind that Rules-As-Written, it is not the player that decides to take a long rest, but the DM/GM that decides if a long rest is available. Make sure you know how your DM is running the game, because you cannot initiate a long rest without DM approval, and that may determine how you use your spells and which spells you prepare.