Does the old expansions count as a paid copy of the game?

Solution 1:

If your account is a trial account, then you can't even add an expansion to the account, so it is completely moot. You need to subscribe, which gives you a non-trial account which includes all expansions through MoP. The unused Wrath of the Lich King expansion is effectively worthless. That key alone will do nothing for a trial account (or a regular subscriber account for that matter, unless it's a special deluxe edition).

Don't underestimate the Blizzard support. They can do anything to your account. I had issues in the past with multiple Warcraft accounts on one login, and they were able to move characters around, including a "Recruit a friend" mount reward, disable old accounts, etc.

Solution 2:

A simple expansion key does not count as a paid copy of the game. As written here

all World of Warcraft subscribers will now automatically have access to all of the content and features of Mists of Pandaria, even if they never purchased the expansion, at no additional cost.

The key info is that you have to be a "subscriber" to get the free expansions. The page here have a definition of subscriber :

World of Warcraft subscribers include individuals who have paid a subscription fee or have an active prepaid card to play World of Warcraft, as well as those who have purchased the game and are within their free month of access. Internet game room players who have accessed the game over the last thirty days are also counted as subscribers. The above definition excludes all players under free promotional subscriptions, expired or cancelled subscriptions, and expired prepaid cards. Subscribers in licensees’ territories are defined along the same rules.

So if you get back with your old account and pay to play, it should upgrade you to Mists of Pandaria for free.