Although they share the same main ingredient, almond butter and almond paste are different enough that they shouldn't be used interchangeably in most recipes. Other ingredients, however, can be successfully used in place of almond paste most -- if not all -- of the time.
Almond butter and almond paste differ significantly in taste and texture. There are two ingredients – sugar and oil – to thank for this. Almond paste has a much higher sugar content than almond butter, and is featured in sweet baked goods such as tarts and filled croissants. It has a stiff texture compared with the creamier almond butter and, more importantly, almond paste holds up during baking instead of melting. Almond butter melts because it contains more oil or other fat than paste. Attempting to swap one for the other will most likely lead to unhappy results.
What can you use as a substitute for almond paste? The next best option is marzipan. Marzipan is a soft, pliable candy base that has the consistency of sculpting clay, and is primarily used to create three-dimensional decorations for cakes and other desserts. It is made with a base of ground almonds, to which is added corn syrup and flavorings. Because it is sweeter than almond paste, reduce the quantity by as much as one quarter to avoid an overly sweet result.
Sweetened condensed milk flavored with almond extract is an alternative when a recipe calls for a very small amount of almond paste, such as a teaspoon or two. Keep in mind that it will melt more than almond paste, but the sweet taste will still be more similar than the earthy flavor of almond butter. If you have them, grind some fresh almonds and add them to the condensed milk. This will thicken the consistency to more closely replicate the paste.
If All Else Fails...
If almond butter is all you have, doctor it a bit before using it in place of almond paste. Add a small amount of powdered or superfine sugar to the almond butter and combine well. This should both thicken and sweeten the almond butter. If available, add a drop of almond extract and mix it in. Finding the exact formula might take some effort, but the ability to recreate the sublime taste and texture of almond paste is time well-spent.