Yes — an upgrade takes effect immediately, so you don't wait for the next billing period. The higher plan's limits and features open up on your account as soon as the payment goes through.
A downgrade behaves the opposite way, and this is the part worth planning around: it takes effect at the next billing period, not on the spot. Your current, higher limits stay in place until then, so you never lose capacity you have already paid for — but you also cannot shed a tier mid-month to stop paying for it sooner.
What "immediately" covers on an upgrade: an exhausted weekly offer allowance is lifted at once, with no wait for the Monday reset, and the higher plan's features become usable on the demand and offer screens straight away.
On the money, you are billed only the prorated difference between the two tiers, not a fresh full price — so upgrading part-way through a period is not penalised. Whatever tier you land on, MarbleMap still takes no cut of your sales; the platform earns from subscriptions alone.
You can confirm the plan that is actually in force at any time under Settings → Subscription.
Also asked as: When does an upgrade take effect? · Do I wait until the end of the period for new features? · When does a downgrade kick in? · Am I charged the full price when I upgrade mid-period?