Hi all
First post so be gentle please 
I have just had to replace an alternator on a yacht but the only available one was an upgrade from 50 to 80 Amps. I have a 50Amp DC-DC charger in situ for the lithium house bank but do I need to change this to an 80Amp one ?? My thinking is that leaving it at50Amps is actually safer as less likely to overheat at low revs ??
No, you’re safe with your 50a charger, no need to spend more money.
No reason to upgrade the charger unless you want to take advantage of the extra alternator capacity. Personally I think that’s a good combination what you have there, the charger cannot cause excess heat/wear on the alternator.