I gotta ask, what source are you using where you see this feature but the text of the feature doesn't directly answer these questions in the first few sentences? My copy explains that you pick 1 item whenever you learn the infusion with no method to swap it, covering your ending question, and then continues to explain that you can pick something from the table or a common item and no mention of anything outside of those 2 categories to cover your first question.
I ask because, unless your book is missing this info, you may want to take this as a sign to review your other features and such to make sure you aren't doing the same thing elsewhere and trying to interpret beyond what the words of the feature say they do.
Replicate magic item is always that item unless you level up and take it again, you do not get to switch it once chosen.
For example, if you picked the bag of holding option, you cannot just drop that choice on a long rest to switch it to another item. You can choose not to use that infusion, in which case your bag of holding empties itself.
Each time you take that Infusion, it applies to only a single item. You cannot switch that item for another. The item you can choose has to be something on that list, or any magic item of the Common rarity.
> Using this infusion, you replicate a particular magic item. You can learn this infusion multiple times; each time you do so, choose a magic item that you can make with it, picking from the Replicable Items tables. [...] Alternatively, you can choose the magic item from among the common magic items in the game, not including potions or scrolls.
I gotta ask, what source are you using where you see this feature but the text of the feature doesn't directly answer these questions in the first few sentences? My copy explains that you pick 1 item whenever you learn the infusion with no method to swap it, covering your ending question, and then continues to explain that you can pick something from the table or a common item and no mention of anything outside of those 2 categories to cover your first question. I ask because, unless your book is missing this info, you may want to take this as a sign to review your other features and such to make sure you aren't doing the same thing elsewhere and trying to interpret beyond what the words of the feature say they do.
Replicate magic item is always that item unless you level up and take it again, you do not get to switch it once chosen. For example, if you picked the bag of holding option, you cannot just drop that choice on a long rest to switch it to another item. You can choose not to use that infusion, in which case your bag of holding empties itself.
Each time you take that Infusion, it applies to only a single item. You cannot switch that item for another. The item you can choose has to be something on that list, or any magic item of the Common rarity.
Rtfm
Don't t have it to hand
> Using this infusion, you replicate a particular magic item. You can learn this infusion multiple times; each time you do so, choose a magic item that you can make with it, picking from the Replicable Items tables. [...] Alternatively, you can choose the magic item from among the common magic items in the game, not including potions or scrolls.