Cannot downgrade baseband from 6.15.00 to 05.13.00

I want to upgrade my 3GS to 5.1.1 from 4.1(8B117). my baseband is 06.15.00. i have tried to downgrade my baseband using 'redsn0w_win_0.9.14b2' and 'redsn0w_win_0.9.13dev4' a million times but it just doesnt happen. i am still on baseband 06.15.00 even after all these tries. in redsnn0w it says before 940 its oldbootroom and after 945 its new. my serial is **941******. So, i have tried to select 'new' as well as 'old' bootroom either way it doesnt work. PLEASE HELP!!HELP!!HELP!!