Learn Swedish: Till skillnad från eftersmaken förändras aromen under rostningen. - Unlike the aftertaste, the aroma changes during the roasting.

Yes, you could say I motsats till eftersmaken. Both expressions mean "unlike" or "in contrast to" and are largely interchangeable. Till skillnad från is very common for comparing two things directly, while i motsats till can sometimes sound slightly more formal.

Why is the verb förändras placed before the subject aromen?

This is the V2 (verb-second) rule. The entire contrastive phrase Till skillnad från eftersmaken acts as the first block of information in the sentence. Because the verb must come second, it forces the subject aromen into the third position right after the verb.

The verb förändras ends in an s. Is it a passive verb here?

It functions more like a "middle voice" verb here. While the active verb förändra means "to change [something else]", adding the -s makes it intransitive, meaning "to undergo change". The aroma isn't necessarily being actively changed by an outside agent; it is simply changing on its own.

Why are eftersmaken, aromen, and rostningen all in the definite form?