Q: Will I have access to all my past games if I subscribe to a premium membership?

A: All your past games are saved if you have a premium subscription, but as noted on the premium subscription page, past games older than 30 days are automatically deleted for free accounts. Which means if you have finished a number of games while using a free membership, they are likely to be deleted after 30 days and will no longer be available even if you subscribe later on. Unless your opponent has a premium membership, and the games were saved for them.

In other words, the only guaranteed way to have access to all your past games, is a premium membership — all your past games are saved on the server indefinitely (for as long as you maintain your premium subscription).