Great question! The Guild perks modifier is applied when the pack is opened, so any pack you hold onto until your Guild reaches level 38 will be affected by the higher chance to receive a Legendary unit.
(Please keep in mind that the 50% chance increase applies to the base stat. For example, if there is a base 2% chance to receive a Legendary unit, 50% of 2% = 1%, meaning you would now have a 3% chance to receive a Legendary unit, NOT a 50% chance at receiving a Legendary unit.)
Hope that helps!