Are you sure you don't mean the older 5400 or 5400HS (D) versus the newer 5600HS (D)?
The older 5400 flash models are not compatible with digital. So, my guess is that's what you meant.
You need the newer 3600HS (D) or 5600HS (D) for use with KM or Sony DSLR models. Sony also has these same flashes rebranded with new model numbers. They're the HVL-36AM (same as the 3600HS (D)), and HVL-56AM (same as the 5600HS (D)).
There is no good way to trigger the old flash models that I'm aware of and control their output (they will always fire at full power with digital and I don't even know if you can trigger them via wirelss using the camera's flash with digital).
From a cost perspective, you'd probably be better off using non-dedicated Auto Thyristor type flashes (Sunpak 383 Super, Vivitar 285HV or equivalent) with radio triggers. These types of flash models are relatively inexpensive in the used market.
I spent a total of $48 for a flash system to use with my Maxxum 5D (and that included two external flashes).
* $16 for a third party adapter to give the 5D an ISO standard hotshoe (and a PC Sync Port).
* a Sunpak 222 Auto with tilt and two aperture ranges for a smaller flash unit (GN of about 72 feet at ISO 100) for $7.00 from KEH.com (and they even threw in a nice, coiled PC Sync Cord with it).
* a Sunpak 333 Auto with tilt, swivel and zoom head with multiple auto aperture ranges, as well as better manual settings (full, 1/2, 1/4, 1/8, 1/16).
GN runs from 86 feet to 120 feet at ISO 100, depending on the zoom head setting. I got this one for $25 (like new in box in 10 condition from the used department at B&H).
Total Flash System Investment: $48
You can get an adapter that gives these cameras an ISO Standard shoe. It's the Minolta FS-1100 (sold out at most vendors now). I use this third party adapter instead (I have a Sunpak 333 Auto and Sunpak 222 Auto I use with my KM 5D and Sony Alpha 700).
You can also find radio triggers that you could use with non-dedicated flashes that have an ISO standard foot (put a transmitter in the hotshoe adapter, receivers on external flashes).
Then, either use the flashes with one of their Auto ranges, setting the flashes and camera to match for aperture and iso speed (that way, the flashes terminate their own output when they see enough reflected light for the settings you're using with their built in sensors), or just use their manual power settings.
This same vendor sells radio transmitter/receiver kits at very low prices. But, I have no experience with their radio kits.