Can I substitute ingredients when making cocktails?

Whilst I did say "it will always work", don't take me too literally.. if you change whiskey for another whiskey, the cocktail will still work. London dry gin for a contemporary gin, it will work... London dry gin for Sloe gin (which is a fruit macerated and sweetened gin) on the other hand, might need some tweaking. Introducing agave in place of simple syrup, it will work but you'll need to tweak the measurements. You get the idea :-) My point is - experiment... tweak... taste and learn about the ingredients.