Just for clarity, if to OP is looking to upgrade to WorldBusiness Class on an intercontinental flight, the only upgradeable fare classes are Y and B, and either of these will give 150% EQM whether upgraded at time of purchase/ticketeting or any other time before travel. The only time it would make a difference is when looking at most travel within North America, as more fare classes are upgradeable and not all offer 150% EQM.

Assuming the OP is talking about intercontinental flights, then be warned that upgrade space is very limited on these flights and you're best off to purchase the upgrade during the reservations process to ensure you get it.