I was a subscriber before the new system went live, so I’m grandfathered in the old system during the transition. Can I use the Loot Coins I’ve already received to redeem the Legacy Rewards earlier?

Yes, you can, although you will get all the Legacy Rewards relative to your active subscription(s) naturally, so it’s advised to save your Coins to unlock other rewards you wouldn’t get just by subscribing, unless you are thinking of canceling your subscription.