But they have the grenade launcher for anti personnel so why replace with 23mm lol.
That is the question... a grenade launcher would compliment a flat shooting high velocity gun but the muzzle velocity of the 30m grenade launcher is less than 200m/s... there is a new grenade that will reach 2.1km or so but most rounds go to about 1.8km or so.
The 14.5mm is a threat to light armour out to about 2km including aircraft and fires its rounds at 1km/s, but then the 23mm round is a similar size but of much larger calibre.
Previously I would have said the difference is that the 23mm round has an enormous and much more effective HE payload travelling at about 700m/s which is not super flat but certainly is not a rainbow either. The extra mass means more momentum so while it will slow down fairly rapidly while supersonic when it gets to subsonic speeds it will tend to travel much further than a lighter round going faster initially.
The point is that it is a flatter shooting round and should be able to reach to much greater ranges than the grenade launcher so it could possibly be worth it at a time when the 14.5mm is starting to struggle with full calibre rounds to penetrate a lot of targets on the battlefield because most HATO vehicles are designed to be resistant to it.
The shift however is with the APFSDS rounds for the 57mm and 30mm calibres of different types we have seen including 57mm grenade calibres.
An APFSDS round for the 23mm would be even more powerful than a similar round for the 14.5mm calibre... the larger diameter tube means it could operate at higher pressure and push more energy down the barrel, so you get a heavier projectile/penetrator and a faster moving dart.
The problem is that you need separate belt feeds because the trajectory of the HE round is going to be totally different than the trajectory of the APFSDS round so a mixed belt would mean one type of round hits and the other type misses completely. Changing between belts means changing trajectory scales or ballistic tables... which makes sense anyway simply because neither round would be multi purpose, so you would fire one or the other but not both and the presence of the 30mm grenade launcher means soft targets can be engaged immediately... so I suspect most of the time the APFSDS rounds would be loaded and if there is no time to change then the grenade launcher is used... a twin belt feed mechanism that allows push button feed and push button changes of feed with the separate elevation of the two weapons you could in fact line up the target and calculate angles and have both weapons elevate and start firing with the grenade launcher at a target at 1,500m and then change the KPB to 23mm HE in a second or two and then start firing with that too and the 23mm rounds would probably still land first...
But they have the grenade launcher for anti personnel so why replace with 23mm lol. I always thought the brdm-2 needed a grenade launcher along with its 14.5mm simple things like this can make a difference. I know some don't like it see OT as out dated and prefer mrap and other armoured 4x4 but for me these are still useful and they have one advantage over the armoured 4x4 and mrap and that's that they swim proper amphibious
I totally agree, the 30mm would fully compliment either a 23mm gun or a 14.5mm gun, and those upgraded models recently sold to a CIS country with the bolted on extra armoured plates... I reckon they could go a step further and make them proper box tiles that contain cavities and ceramic armour to massively increase protection but also boost buoyancy to perhaps even improve amphibious potential.
It also had modern night vision equipment, but I don't think they fitted a grenade launcher.
I like the fixed model fitted to the upgraded BMP-2 at the back of the turret that can elevate but doesn't need to traverse... but the BRDM-2 turret is not big enough to fit it centrally as it would interfere with a centrally mounted main gun.
I would think and enlarged turret with the entire rear half being a magazine for 30mm or 40mm grenades... you could get 600 in there if you wanted to... with light armour skin front and back and isolated from the turret inside with a 40mm or 30mm grenade sitting on the top of it offset to one side so it is not shooting grenades into the barrel of the main gun when the main gun is elevated and the grenade launcher is shooting at closer targets.
Effectively the current rear of the turret armour would have a tiny 5mm air pocket and then the armoured inner skin of the ammo bin and then the grenades loaded in one continuous belt... and then an outer skin that is rifle calibre bullet proof... and then a gap of 20mm and then another 15mm thick plate to protect it from small arms fire. Ceramic panels could be slid down the 20mm gap if required.... and a metal cage frame around the rear that contains smoke grenades firing forward that you can strap gear to if you want as well...
I could use it for hunting in areas with rivers and lakes...

Do they use the belly wheels much?
I would think I would talk to an experienced operator to find out how often they are used because they would add weight and complication... if they never get used I would get rid of them and increase the internal height space inside the vehicle... perhaps keep the same height and use it for fresh water or buoyancy... ropes or tools or equipment...