The purpose of heavier wiring is two things. First, to lower the voltage losses in the system. Second, to safely carry the current that the sytem will be running at.
If you have a short run of cable, i.e. two feet of a ground cable, and it is heavy enough to carry the current without overheating, you may not care about voltage loss since that is based on linear feet of cable. On the other hand, if you had fifty feet of cable running forward to a windlass--you'd NEED the heavier cable as fifty feet of the same "per foot" voltage loss could be crippling.
Do you need to upgrade it all? Probably not, but you need to take a look at each bit and see if it fills both criteria (carrying capacity and voltage loss) as it is. If you are doing a whole system, you may find that buying cable from a 100' spool, or from a spool "end" surplus, is way cheaper than buying it by the foot, and there's enough to replace everything.
But the cable fittings, the lugs, etc. are equally critical. The wrong ones (like the clamp-on ones) will cause voltage loss that you don't need. Crimped fittings need to be crimped properly, not just banged on. So as you spec all the cable runs, also spec each fitting and you may find that the costs of the fittings are as much as the cable is, and for short runs keeping the older smaller cable--with fittings--is a big cost saver without impacting performance.
But either way--the cable size has to be judged by the load and the wire length, you'll have to go over the numbers for each piece to see how critical it is.