Hi! 

If another Alliance manages to perform a successful attack on your HQ while it is being upgraded, the Upgrade will fail, and your HQ will be downgraded by one Level. Your Alliance will be reimbursed 75% of the Construction Packs spent on the attempted upgrade.

ALASKA FROST said:

My hq is level 5 we have a enemy that keeps attacking us if i shell out the diamonds needed to upgrade to level 6and they attack during this process and my troops can handle the attack does it still get dropped back to level 5 or once upon reaching level 6 can it get dropped to level 5

if you stay a level 5 it can't be downgraded again, 

my advice is to save your resources and just build up on troops and make sure you get it well defended 

when PVP and EXP and downgrade challenges are about your team isn't spotted off for the level of your HQ

and it might save you troops and resources you had used, they are a high priority in the game...