There are several cards that let you "replace a unit" on board, generally with a "3" strength unit.
Can a "1" strength unit be replaced for this event, or do you need to replace a "2" strength unit, if possible.
We assumed a "1" strength unit was fair game to be replaced and bumped up to a "3" strength unit.
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Replacing a 1 or a 2, for a 3 strength is ok.
Just follow the wording on the card, and implement. For these cases the wording on the cards defines the requirements and restrictions.
If it says place a unit when an area is "friendly or contested" then as long as it is not enemy controlled then it is ok.
If it says place a unit in area is "friendly" only, then contested or enemy ownership of that area would disallow the placement, at that time.
If you do not have a unit available of that type and strength then you cannot place the unit, at that time.
There are other event cards that are more specific in the units being exchanged. i.e. a 3 strength CTV for a 5 strength CTV. If you do not have a 3 strength CTV then you cannot carry out the improvement.