Let my give you some sentences: We can use three ways to say:"She looks very young." 她的样子很年轻。 她看上去很年轻。 她看上去很年轻的样子。 We can use three ways to say:"This dish looks very creative." 这个菜的样子很有创意。 这个菜看上去很有创意。 这个菜看上去很有创意的样子。 So could you see that? Sometimes these two words are interchangeable, sometimes we can use the sentence patten"N+看上去+Adj+的样子” to express the same meaning.

They are not interchangeable. "樣子" is a noun. We use "樣子" for appearance, some action you can see. For example, "他的樣子有點奇怪" means "His appearance is weird" or "His action(what he doing) is weird". "看上去", in fact, we use "看起來" more often, is a verb. "天氣看起來不錯" means "The weather seems good".

They are not interchangeable.