If you're subject to retrogression from Oct 1, then your ability to use concurrent filing will be effectively lost on Sep 30 2005. This is the essence of the recent visa bulletin which caused so much chaos due to the unexpected steep retrogression by several years for mainly Indians and Chinese in all categories, and for the entire world in EB-3 category.
Thats why everyone is rushing to file 485 before the Sep 30 deadline. 140 can be applied even after that date, but not 485 if subject to retrogression. If not subject to retrogression, concurrent filing is only an additional advantage ( a huge one for most people) but never a requirement. 485 can be applied even several years after your 140 is applied and even approved, if you choose to do it that way..
But most people seem to want Green cards with the least wait and hence to apply for 485 at the earliest.