A lot of heartfelt answers here but I feel the best answer is to deliver results to members who are not yet upgraded let them see that choosing to upgrade will allow them to scale whatever results they are getting at the free level can be scaled up i.e wider reach for example if am restricted to mail only 1000 members but am receiving a better than expected response from mailing the restricted list then maybe if I upgrade and am able to mail 10,000 instead of 1000 with a 25% click through rate plus sign ups and sales then who wouldn't be willing to upgrade their membership?
I see where a lot of programs are forcing you to upgrade when you actually not getting any result from being a free member - no quality traffic, no engagement, no sign ups and no sales so why should I upgrade? If your program delivers the sh*tty when it's free then why should members assume if better if I upgrade since am not receiving any good results being a free member?
On the other hand if upgrading to GOLD has all the whizz and bang effects and giving upgraded more than they expected then give the free members a taste of what the "good life" if they upgrade by offer a short trial of the upgraded membership for example a Thirty (30) Day trial option before they are charged for an upgrade.
If your trial offer of the premium level of your program is in fact delivering more than the expectation of the members then the chance of getting them to upgrade is escalated since all will agree that your program is worth paying (since the value benefit exceeds the cost by perception).
If you offer a free trial of your premium membership and no one continues with the paid option then it's your fault your program sucks.