Changing a market's keywords
Add or remove keywords at any time. Keywords from the database bring their history with them, and every average is recomputed across the new set.
Open the market and edit the keyword chips in place, or use Edit market. Changes save themselves.
What happens to history
Keywords you keep are untouched.
Keywords you add from the database arrive with their full history, because the database has been capturing them all along. From the next night on they are captured daily, like the rest of the market. A keyword that is new to the database starts from its first capture; see Keywords that aren't in the database yet.
Keywords you remove leave the market, but not the database. Add one back later and its history is still there.
The averages follow the set
Every average is computed across the market's current keywords, for the whole period you are looking at. After an edit, a chart can change shape without anything having happened in the market: the past is now measured on a different set.
That is not avoidable, because an average over a set has to pick a set. If you want a before-and-after that stays comparable, change the keywords in one deliberate move rather than a few at a time.
The field follows the keywords
The products in a market are the ones that rank on its keywords. Adding terms brings in whoever ranks on them; removing terms can drop products that only appeared there. See Who counts as a competitor.
Limits
A market cannot grow beyond your plan's keyword limit. If it is already larger, for example after a change of plan, you can keep it as it is, shrink it or swap keywords, but not add to it.