Kendra Little explains how index modifications affect the plan cache:
Creating an index doesn’t remove a plan from cache
We run our Demo Query five times. Querying sys.dm_exec_query_stats and related DMVs, we see five executions, plan_generation_num = one.
Then we create an index on a table referenced by the query.
Querying the DMVs we still see five executions, plan_generation_num = one, and the query plan in cache. It hasn’t been removed!
Read on for more, including what happens when you run the query again, what happens when you rebuild indexes, and what happens when you drop an index.