If I’m using a rekordbox ver. 6 subscription plan, can I keep using it even if I switch to a different account that I don’t have a subscription with?

The subscription plan is associated with the account you signed up with. If you switch to another account, you'll also switch to that account's subscription plan. So, you won't be able to use your subscription plan if you switch to an account that doesn't have a subscription.

If you switch to an account that has no subscription plan associated, you can use the rekordbox Free plan. Free plan includes EXPORT mode and selected features in PERFORMANCE mode.