If I have a paying sub who I give 6 months complementary membership to, will it just pause billing for 6 months and the resume billing or will it override it and cause issues where the member will have to opt back in to their paid membership?
If it doesn’t behave like this, I feel like it should.
Bumping this because its so important as it could cause huge problms later not knowing
After the six months, your member would need to go back to the portal and actively upgrade again, according to the docs:
When a complimentary plan expires, members will need to upgrade from within their Portal account settings to maintain access to paid content.
Not sure if that’s an issue – I rather see it as good UX practice, where you avoid that a member gets charged without them remembering about it 
The reason I ask is I integrated viral loops referrals into my website, and they get different rewards for hitting certain milestones. Those rewards are premium access to different paid memberships I have.
Is there a way to grant complementary access for a time period and then billing resume automatically if done directly within stripe? My members are aware or they are told if they are on a paid membership, they will resume being charged when their reward (Free Paid Tiers for x amount of time) has expired.
I think the solution is issuing a credit to their account so when they are charge it is take from their credit balance instead of charging their card. Have you ever used that feature and if so is it pretty strait forward?
I will set up an email that informs them they have a credit for x amount due to their referral reward and once the credit has been used regular billing will resume.