Yes, but it's not perfectly smooth
Currently, as described, there are three ways to drop an infusion.
- You can die, and wait a few days. This will cause you to drop all of them all at once, and also requires you do die and stay dead for a few days. It's not a perfect solution.
You can infuse enough items to exceed your maximum, thus causing your oldest to fall off the back end. Since you can keep touching and keep reinfusing, you can easily decide whichever distribution you like every day. Just re-infuse until you've dropped the ones you want to drop. This is potentially problematic in the case where you have infusions (like your favorite Bag of Holding) that you don't want to drop for even a moment, but otherwise works quite well.
On level-up, you can unlearn one infusion in favor of another, which instantly drops the infusion on whichever one you unlearned. This has obvious limitations on frequency of use, but does let you drop any one infusion cleanly without disrupting any other.
On the other hand, this is UA
UA is playtest material, and they generally won't have caught everything. They haven't considered all of the edge cases. If this is a real issue for you, you can, and should, consult with your DM about it (who should be paying careful attention to balance issues on all UA material to begin with, and adjusting as necessary for their personal campaign) and ask for the ability to decide which infusion you drop when infusing past your max at the end of a long rest. It would be a convenience increase without meaningful power increase. As a DM, I'd permit it.
To infuse an item, you need the nonmagical version of that item.
The artificer's Infuse Item feature states (TCoE p. 12, E:RftLW p. 57, WGtE p. 179; emphasis mine):
At 2nd level, you gain the ability to imbue mundane items with
certain magical infusions. [...] Whenever you finish a long rest, you
can touch a non-magical object and imbue it with one of your
artificer infusions, turning it into a magic item. An infusion works
on only certain kinds of objects, as specified in the infusion’s
description.
The Artificer Infusions section at the end of the class description states (TCoE p. 20, E:RftLW p. 61, WGtE p. 181; emphasis mine):
The description of each of the following infusions details the type
of item that can receive it, along with whether the resulting magic
item requires attunement.
And finally, the description of the Replicate Magic Item infusion states (TCoE p. 22, E:RftLW p. 63, WGtE p. 182; emphasis mine):
[...] Alternatively, you can choose the magic item from among the
common magic items in the game, not including potions or scrolls.
In the tables, an item’s entry tells you whether the item requires
attunement. See the item’s description in the Dungeon Master’s Guide
for more information about it, including the type of object required
for its making.
Each of these portions I've bolded make it clear that you need a nonmagical version of the item to infuse, whether you're using Replicate Magic Item or one of the other artificer infusions.
Even though the Dungeon Master's Guide doesn't actually say "[X] is the type of item needed to make [Y]" for any listed magic item (as far as I know), this sentence seems to refer to the fact that each magic item that can be replicated is essentially an enhanced version of a clearly identifiable mundane item.
For instance, boots of elvenkind are boots, so the clear implication is that you would need mundane boots to infuse if you want to use Replicable Magic Item to make boots of elvenkind. This follows the same logic as the Boots of the Winding Path infusion (TCoE p. 21, E:RftLW p. 62, WGtE p. 181), which does explicitly state:
Item: A pair of boots [...]
In the case of your example, the description of cast-off armor states (XGtE, p. 136; emphasis mine):
Armor (light, medium, or heavy), common
Thus, you need a nonmagical version of whatever kind of armor you want to turn into cast-off armor. If you want cast-off half plate armor, you need a nonmagical set of half plate armor to imbue with that infusion.
The Replicate Magic Item infusion doesn't create a magic item from thin air - it gives you the ability to imbue a certain mundane version of that item with the necessary magic to function as that magic item.
Best Answer
Only Replicate Magic Item can be learned multiple times
The Infusions list includes this sentence in its intro:
The only infusion that "says otherwise" is Replicate Magic Item, which says:
Interestingly, it doesn't say you have to choose a different magic item each time, so it seems that you could indeed learn to infuse multiples of the same magic item from this table.