There's a few different ways to go about this, but the easiest/simplest in my opinion would be to buy the new spool bolts from indian(if you don't have them already). From 2019 on they made the quick release latches an option instead of standard, and standard is now a bolt on setup. The new spool bolts have a head that accepts a second bolt thru the bag hole to hold it on. Think the spool bolts are like $7ea or so, another like $5 ea for the second bolts that hold the bag on. Then either set of bag will work just fine on your current spools.
If you want the quick release there are ways to make both bags work with the same size spool. The spool size is mostly for the rack/sissybar to mount to. The outside diameter doesn't matter for the bags, its the inside diameter, and that only matters because the latches for the leather bags have a smaller diameter and shorter length(hard bag sides are thicker so the anchor pin is longer for them than the leather bags). So if you can get a shorter anchor pin with the larger diameter, then they'll both work with your large spools.
But if you just don't want to buy anything else extra, yes you can just swap the spools each time you swap bags. Takes a bit longer but works just the same.
As for them falling off on the highway, I've seen a few posts here about that but never seen/had that issue in real life. Best guess is the quick release latch either wasn't fully latched, or wasnt fully inserted into the spool. Or something in the bag caught the latch and popped it up. My 2018 Vintage had the quick release and smaller spools, even unlatched they sometimes stuck in pretty good trying to get them off. The quick release on the hard bags should theoretically be just as easy/hard to fall off as the leather bags, it's the same latch type system, just a slightly larger diameter. So if you haven't already had that issue with the hard bags then as long as you latch them right you really shouldn't have that issue with the leather ones. My 2019 Vintage now has the new bolt on setup with the larger spools. Bolt on would have to back both bolts completely out before a bag would fall off, really doubt that'll happen.