SC2 help??????????!!!!!!!!!!!!?

so i have an account that is in silver rank 15 (my main account thats already paid for) but i used my guest pass to see what league i would be in if i redid placement and i got plat! not only did i get plat but top 15 in play after playing on it for a day! the guy in first is only 200 points away and i wanna stay on this account. but the trial ends tommorow i want to stay on my plat account due to the fact that i am over 2000 points away from first on the silver account. how can i do this? is it possible without paying the $40 for the full game on that account? what if i sold my silver account online? any suggestions on any way to play on the plat account? thanks for any suggestions/help