You can change these at any time through your iTunes Connect or Google Play accounts. However, keep in mind that iTunes requires going through approval by Apple, so changes would not be immediate. Also, the changes would only affect new subscribers going forward, after it is approved.
Free Trial Period:
This is available on both iOS and Android. You can offer a free trial period of 1 week or 1, 2, or 3 months. It will begin when the customer subscribes and they are prompted to accept. They get free access to your issues during that time period. They will be automatically charged when the free trial is over, unless they have turned off automatic renewal, in which case they will not be charged, and will lose access. For example, for a 7 day trial, they get access for 7 days, and on the 8th day they will be charged for the first month's subscription.
This is available on iOS only. You can offer users a free subscription extension in exchange for sharing their contact information with you for marketing purposes. The reader is prompted when they first open the app. If they say yes, they will get an extension - free access to your publication for a time period set by you - 1 week, or 1, 2, or 3 months. The user’s opted-in info will be made available in the Sales and Trends module of your iTunes Connect account.
Note #1: Free Subscriptions can’t offer a marketing opt-in incentive - since the subscription is already free - but users will still be asked if they want to opt-in to share their information.
Note #2: Publisher's download fees still apply during this period.