I signed up for a subscription (Virtuoso) and I went to change it to Superstar… but it won’t let me because I am still in the free trial period.

I ended up getting another subscription (which I planned to cancel once the 15 days are up)… but I need access on more than 2 devices.

Can you please help me upgrade my virtuoso to superstar and cancel the other one?

Sure. Please write to support at volumio dot org and we will take care.
In any case I suggest to cancel your virtuoso sub and start superstar