UPDATE:
GREAT NEWS
https://forums.insynchq.com/t/update-free-prime-promo-and-upgrade-paths/7789
Thank you.
Hey Guys,
i am a long time payed user and now see that only new users can get FOR FREE Prime.
Thats not fair and a kick in the ass for all pro users.
I’m tempted to cancel my subscription and see if I can get register for the two free Prime accounts that InSync has sent me in an email with holiday greetings.
Not entirely sure how this translates into “Giving away for free!” Below is the exact mail I got from Insync after signing up with my gmail account:
Welcome to your Insync Prime trial!
We have 15 days to convince you to buy a license so do abuse your free trial with these advanced features :)
Our Happiness team will email from time to time to check how you're doing so don't be shy :)
Glad to have you on board!
Insync TeamHere we go, new mail from insync this morning. It seems that it really is free. As I am a plus user on my main e-mail, I signed up for a plus on my backup gmail account, which apparently worked 
This is to confirm that you've successfully redeemed your free Insync Prime from our promo. Please disregard any previous email that was sent to you about being on a trial.
Don't forget to share the gift with others :)
Happy holidays,
GioWe’re sorry for the inconvenience this has caused our customers. Rest assured we are resolving this and will be sending out an update soon.
I sure hope that the “update” will be that current pro and plus members (your paying customers so to speak) get a free upgrade to prime. It is downright [censored] your paying customers by offering “prime” for free to anybody who walks into the door, but expecting your current paying customers to cough up extra money to be able to upgrade to prime. Do you have a CSO (Chief Stupid Officer) in your management team who comes up with brain dead ideas like this?