You don't specify what first stage you have. If you have a Mk 10, and are using tanks with service pressures less than 3300-3500 psi, a DIN conversion kit will do the job just fine and there is no real advantage to getting a new first stage as frankly a new first stage will not do the job noticeably better than your Mk 10.
If you use higher pressure tanks and have a Mk 15, MK 16, Mk 17, Mk 18, Mk 20 or Mk 25, there is no benefit at all to getting a new first stage as all of them will work fine at service pressures up to 4350 psi.
All of the above regulators use the same universal 300 bar DIN kit. You need part number 10-045-030 and the dealer cost is about $35.00-$40.00.
As for a yoke to DIN adapter, you can't get one suitable for underwater use. There are some out there, but they are designed for use on fill whips.
Once you convert to DIN, you can get a DIN to yoke adapter. Scubapro sells one that comes with a new yoke, but since you already have a leftover yoke, you can also get the adapter without the yoke for about $25.00 less.
If it were me, I would not put the old G250 on the mantle, because they actually breathe as well as anything else available today and they do it with a metal air barrel that produces less dry mouth. Assuming your old G250 has been properly tunred, you'd be less satisfied with a "new" second stage.
