The front derailleur and chainset on my bike has served me well since the mid 90s but increasingly erratic shifting has led me to decide that I should probably replace at least the chainrings, as they do seem to have something of a sharks fin profile to the teeth, which youtube tells me is not good. My main question: If I replace the chainrings, and/or the whole crankset, how much do I need to worry about compatibility with the mech, and the rear derailleur, and/or anything else? Is it pretty much anything goes, or are there things I need to look out for? If I get chainrings that are slightly different sizes from the existing ones does it matter (I fancy swapping the big one for a bit of a larger one)? If it makes any difference the existing is Shimano Altus C20 and the rear derailleur is Tourney TX (I replaced it at some point).