Packages (bundles & passes)
Yes, you can edit packages from Packages > click on package from appropriate filter > Edit this package. Only tag and purchasable object (i.e. what it can be used with) changes apply to all new and existing bought instances of the package. Editing all other fields apply to new purchases only.
Purchased Instances (bundles & passes)
When you update the tag or purchasable object (i.e. what it can be used with) on the master package this is applied to all purchased instances by default. You can also, edit the other fields like expiry date and number of credits on a purchased package. You can easily do this by going to Packages > click on specific bundle / pass > Package customers > tick the checkbox next to the customer's name(s) > Update > from here you can add or remove credits and update the expiry date or expire now > Next > Update package.
Whilst you can't freeze a package indefinitely, you can amend it's valid to date at any time, so you can always adjust expiry dates as needed.
Memberships
You're not able to edit individual purchased instances of a membership, however you can change usage limits or tags within the membership itself which will apply to all new and existing purchased instances of this package. You can easily to this by going to Packages > click on package from appropriate filter > Edit this package.
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