If I was to make a mistake that leveled my hp would I be able to recover it back to 9?
The tricky part with 9 hp accounts is once you level your hp, we have no way of knowing that you previously had lower than the current minimum Constitution XP. The evidence has been erased, so to speak, by you now being 10 hp +.
From what I've been given to understand, 9 hp accounts were created in the transition from Classic to RS2 on 8th March 2004. That's the only point of reference we have is your account is now over-levelled.
So how I've handled it is that *if your account creation is earlier than 8th March 2004, resetting your Constitution sets it to 1,000.0 XP*, instead of the usual minimum of 1,154 XP. (Applies to both the major Constitution & Prayer reset and minor Constitution reset.)
Resetting Constitution consequently may create 9 hp accounts that weren't active for that transition, or if your account is newer than that and somehow got 9 hp through some other event, then you'd be reset to 10 hp.
I'd be happy to tweak the logic further if ...
Read more