Interesting question. Here’s my take.
Extending species like Mountain Hemlock, Blue Altas Cedar etc normally are best repotted at the cusp of the growing season. This is the time the tree is very most responsive to growing.
In the case of the former it’s when the buds begin to turn green. For the latter, when the buds begin to turn blue green. In fact pretty much most operations can be done in a roughly two week time frame at this trigger point.
The rationale behind this statement is root and bud growth on these trees begins after/as the buds push and accelerates through the summer. Energy to fuel growth of both the new needles and the roots initially comes from the already existing needles starting peak production for the year and stored energy in the trunk etc.
It is certainly true a height of the summer repot can be done of BAC if the tree is properly managed afterwards. Totally understand the rationale. Yet doing so loses half of the strongest root growth during the growing season.…and will take away some water and mineral resources, slowing growth in the top hamper.
Performing a repot before this trigger point would certainly be ok, yet the trees roots will languish…growing slowly… until the trigger point arrives.
cheers
DSD sends