Zyte Smart Proxy Manager(formerly Crawlera) plans can be upgraded or downgraded anytime, it would not affect the existing API key. API keys are mapped to the account and not to the plans. Any upgrades or downgrades would result in Concurrency and Monthly request limits. \
It can be changed by clicking on "Generate New Smart Proxy Manager Key" in the Smart Proxy Manager setup page.
For any queries, do reach out to our Support team.