Stand-alone purchase is different than an Object Desktop subscription, as it appears you understand. When the release version is ready (should be soon given how long the beta has been out) you'll be eligible for upgrade, either free (if your purchase was recent enough) or at a discount.
I'd spring for the OD subscription & eliminate the wait, but that's me.
Edit to clarify (I hope) -
Historically, SD has provided free updates to interim dot-x releases of the same base version (e.g., 6.0 thru 6.99) and offered upgrade discounts to existing standalone owners when a new base version was released (e.g., discount on 7.x for 6.x owners). In some cases, they've offered a free base version upgrade if the prior version was purchased within a defined recent time window. I'm not speaking on behalf of SD, however, and those policies are always subject to change (and may actually have changed) without my being aware as I'm just an OD subscriber.