I suspect golf cart upgraded package to lithium looks like something this. Now if buy new cart it may have just big box containing cells (no battery packs)

Allied Lithium Batteries for Golf Carts 72V (Includes Lithium Charger)

Price wise not that much for conversion plus labor?? After conversions cost would be cheaper just for batteries?

So, individual battery with bad cells be replaced without replacing the hole battery package. And I suspect the bad returned battery probably have the ability to repair cells inside by manufacture? So agreed, repair by individual could not be possible on high voltage battery sealed by manufacture , plus where would get the cells to even attempt it. but replace the bad battery in pack would certainly be option if one when bad I hope?

How do lithium "aftermarket" sellers deal with the weight difference when upgrading AGM or lead acid to lithium on an existing golf cart?

The difference in weight for lithium is enormous (lithium is multitudes lighter) and an electric golf cart needs that weight to prevent tipping if it wasn't initially designed for lithium. DO they add "ballast" to make up for the weight loss? The heavy lead batteries weight is part of the engineering of the GC. You can be talking hundreds of pounds for standard batteries vs (just guessing) under 50 for lithium. My lithium "cart" uses a 60V system which gets converted to 12V for the heat, a/c and other stuff. The motor is direct drive 60V.

The latest Samsung SDI battery cell data sheet (Rev7) states 80% capacity after 1000 charge cycles...So 48 miles after 60,000 miles instead of the 60 miles when new. The capacity fade is pretty linear.

At 6 cents per mile less for EZGO 4.2 electricity than quietech gasoline...$3600 savings over 60,000 miles for battery replacement.

Oh, zero maintenance...Just replace tires on the EZGO RXV Elite. That's at least another $1500 savings over gas cart maintenance.

Yamaha will have their lithium cart out in a couple weeks. I suspect it will have at least a 60 mile range.

That system is a 72v system. I don't know why anyone would want that. Most of the carts in TV are set up as 48 volt systems. From what I've read, which is limited and I'm far from an expert, the systems that go in in pieces, that look like replacement LA batteries, work, but are more prone to issues, as the computer technology that makes them safe are duplicated in each box, and hence, increases the chances of a bad one.

There are several companies that make one piece "drop in" conversion batteries, RoyPow being one of them. I was going to go with a 105 ah RoyPow. STM Carts in Ocala was going to pick my cart up in TV, keep it for 2 or 3 days, do the conversion, and return it to me for around $2700 plus tax. I was signed up to do it. My new LAs cost over $1,000.
I had read that the unit I was going to get was a great unit, but it didn't fit into my Club Car DS without modifications to the battery frame. STM Carts said they had no problems. Then, out of the blue I got an email that said they couldn't do it due to "availability issues" with the battery. I emailed RoyPow and they said they had plenty of batteries, and had, in fact, just delivered a bunch to STM in Ocala. So I am suspicious they didn't want to deal with the battery frame issues.
For $2700 vs $1000 I'd have taken my chances with the RoyPow lithium unit. I wanted the cart running ASAP, and was running into a wall trying to find someone to do the conversion, and did not want the multiple box system.

Elite lithium
 

We love the quiet features of lithium. Range wise we can go from Spanish spring to Fenney and back on full charge in good weather.

With lithium you have to be more mindful and take weather and distance into consideration much like our Tesla.

Colder weather below 50 degrees you loose distance.

I would by it again and looking to buy a second but not for everyone