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.