You cannot upgrade a 3GS made newer than week 35 of 2011 with the 06.15.00 baseband -- it will brick it and the phone's cellular radio/modem will stop functioning. There is no currently known way to upgrade the new 3GS to the 06.15.00 baseband without bricking it.

New Bootrom/Old Bootrom has nothing to do with the week 35 of 2011. New Bootrom/Old bootrom is relating to a change that happened several years ago. Many people are confused by this but I can't be any more clear than the fact that your bootrom has nothing to do with whether your phone can be upgraded to 06.15.00, and the week 35 of 2011 has nothing to do with your bootrom revision.