Is there a grace period to make changes after purchasing an NCE subscription?

Yes — Microsoft provides a short window after purchasing a new NCE subscription during which changes (including cancellation) can be made without the restrictions that apply later in the term. The exact length of this window can vary by product and has changed over time, so it's best confirmed with your partner at the time of purchase rather than assumed.

Outside of this window, changes are governed by the billing term's standard rules (monthly subscriptions renew/adjust each cycle; annual subscriptions are generally locked until renewal, aside from adding seats).