I'm currently working on both!
Just mail the WoWBillingService saying you want a downgrade and they'll do it for free.
I'm getting my account downgraded within a few days and now mining to 450 on my 60 priest - the grind is very timeconsuming but not too bad if you like grinding and you got some nice music and good weed with ya
I'm at 445 right now, mostly skilling on Khorium veins. Rich Adamantite are still green to me but practically never gives the skill.
From 375-420 isn't bad tbh, rich adamantite and khorium practically gives a skill every vein, but from there it gets very tough indeed. I was stuck at 437-438 and mined 12 rich adamantite and 7 khorium veins before I got the skill. The next 4 skills came within the next 4 veins though.
I wouldn't be able to say anything about skinning / herbalism, but I assume herbalism is practically impossible seeing you have to go to Northrend? Oh well, I wouldn't know, I know of mining at least - it's time consuming, but well worth it.
Now the question is - will they downgrade my mining to 375 aswell after downgrading the account itself? Some people claimed they wouldn't and some people claimed that they would, but that I'd keep the 60 stamina bonus. I cannot tell you now, but I'll post about it as soon as the downgrading has occured