CAUTION: The following post (or a page linked to) includes or discusses loading data not covered by currently published sources of tested data for this cartridge (QuickLOAD or Gordon's Reloading Tool data is not professionally tested). USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assumes any liability for any damage or injury resulting from the use of this information.
One thing I noticed in looking through Hodgdon's load data is some of the same powders are used in both rounds with 125-grain bullets, despite the case volume differences. This suggested to me that if you put 1½ 7.62×39 charges in the 7.62×54R cases with the same bullet at the same seating depth, you would get above minimum safe case fill and not likely have a pressure issue.
In QuickLOAD, I tried both faster (1680, H322, IMR4198) and slower powders (BL-C(2), and H4895) maximum loads from Hodgdon's list. In every instance, pressures were from about 4% lower to about 14% lower in the larger case with the 1.5X loads. Since the 7.62×54R is rated for about 10% higher pressure than the 7.62×39 is, this should give you at least 70% loading density and would be conservatively safe with pressure.
If you try this, you are doing it at your own risk. I can't guarantee the unknown powder you have in the commercial ammo will follow the pattern exactly. I can only say that it is improbable it will be grossly different. If you decide to try this and the load you have is done with a powder that has poor case fill in the smaller cartridge, watch out for the 1.5X load to fail to hit 70% fill of the space under the seated bullet in the 7.62×54R. Faster powders are not the best choice in the bigger cartridge, but it looks like you should get a plinking load out of the 1.5X combination. Don't expect great target performance and you could expect some significant velocity difference depending on whether the powder is forward over the bullet or backward over the primer.
If the powder is a spherical powder, you may want to use a magnum primer to try to maximize ignition and minimize velocity variation. You should also be prepared to see unburned powder fouling from the lower pressures. Such may indicate you need more than 1.5X, but you need to be good at reading pressure signs or own a Pressure Trace to pursue that safely. I don't believe your pressure will get down to levels that could stick a bullet in the barrel but understand there is also some percent risk of doing that with this approach.
If you decide to try this, be careful and good luck and please let us know how it goes. Remember this is all based on computer simulations and is not tested load data.
