I have a level eight moon druid and would like to maybe change to another type of druid. Can a swap Druid circles ?
No, not under the standard rules
There are no rules in the books to change your class features. A druid selects their Druid Circle at level 2, but there's no rule that says they may change it later.
However, your DM may be more allowing, and allow you to re-spec your character to change druid circles. The DM is entitled to bend the rules in this way. I'd allow it in my campaign, provided the character attended appropriate rituals and so forth. Ask your DM.
It wouldn't be unbalanced as long as you lose any abilities from your old circle, and can't freely swap back and forth (as this would give more versatility than normal).
The Adventurer's League organized play rules allow you to re-build your character, including changing druid circles, at any point prior to adventuring as a level 5 character. However, these rules only apply to official Adventurer's League play and not D&D in general. That said, this rule suggests that changing your character in D&D is not necessarily an unreasonable request.
As a DM, I think circle changes should always be permitted.
We're playing a game, after all, and want to do our parts as DMs to not get hung up about each and every rule that is written or might be implied and could ruin the game for a player who is unhappy with his character - especially if our games won't be unbalanced by doing so.
I've been playing D&D for decades now, and I firmly believe that the only thing that ever ruins the fun is obsessing on the rules. Anyway, changing a player's circle isn't hard. A DM could be cruel about it and make it hard. He could make it so hard that it wouldn't be worth doing, and I've met some who do, but again, it's a game! A game!
Why ruin it for a player?
So what I do is something like this - and it's not JUST druid circles that I do this for, but all kinds of character revamping, like wizards who went transmutation and want to change to evocation, or whatever - I give the character a quest that would be part of the next dungeon he's in, involving his party finding an item he has to attune to - I have no idea what. Maybe a longbow or druidic focus that's been passed down from moon druid to moon druid (or whatever) for generations. After attunement, he would lose all previous circle abilities and start at the lowest level of his new circle's abilities, which is typically level 2.
And bam! Everybody's happy. Afterall, it's a game.