If you're wanting to increase the pricing on a membership that you currently have on sale you would firstly click Remove from sale on the current membership so that no new customers can purchase this at the old price. Removing this membership from sale will not affect any current live instances that your customers have of it.
You would then create your new membership (you can use the Copy an existing package button to use your original membership as a template), making sure you enter the new increased price and then placing this new membership on sale.
If you're wanting to keep these current customers on the old lower priced membership after their current instances have expired, then you would need to place this old membership back on sale when you know when one of these customers are going to purchase it, once they have, you would then remove it from sale again so that no new customers can purchase it at the old lower price.
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