Haroon, as you noticed ops 1.400 doesn't mention any factor, so you don't have to apply any correction..legally. The word "legally" should not be, in my opinion, over-emphasized because it doesn't mean necessarily "safe". Actually, the regulation gives the responsibility all over the commander as Safetypee correctly says.
Microburst, I agree with you that a re-planning is a planning anyway, but this is irrelevant in this contest. Haroon used the word re-planning, but regulations doesn't, according to them you have only 2 scenarios, before dispatch and in-flight; diversion alternate follow the same rules.
FB