For those wondering, VVVF = Variable Voltage, Variable Frequency. Essentially, a power electronics box is used to synthesise the traction motor's supply voltage waveform instead of 'mains' AC or DC from the third rail or overhead wire. There's very little difference between VVVF and Variable Frequency Drives nowadays, VVVF is just the acronym some Japanese manufacturers use. They're technically AC drives because they use brushless (i.e non-commutating) motors.
How hard would it be for Melbourne train network go from 1500v dc system to a 25 kV AC train system?
This has been covered on the 'Page before, mostly in relation to Geelong line electrification. In short, suburban conversion really isn't worth it.
For a conversion, at a minimum you'll need to change the overhead wire insulators to much bigger ones (on account of the 16x greater voltage) and swap out traction power substation rectifiers for mains supply -> 25kV transformers. You can keep the pylons but you'll need to modify it so that the whole thing doesn't go live in a flashover event (higher voltage = larger air gap that current can arc across).
Wires don't need to be changed over when you're increasing the voltage, but you can get away with smaller cables due to the 'skin effect'. If you're putting in new insulators, you may as well re-string the wire with lighter gauge stuff at the same time though. That's a lot of lineside works for the conversion - you'd better get a big benefit out of it for the number of occupations you'll have to do!
The technical benefits of 25kV AC over 1500VDC are basically that for a given load you can have greater distance between substations. The more trains you run over a given distance (and hence power you draw), the less relevant this becomes. If you're electrifying country mainlines or building High Speed Rail, 25kV AC is the way to go. For commuter and metro rail networks, it's less relevant because of the service frequencies involved.
In most cases, as long as you're using overhead wires for supply, you can use any AC or DC legacy electrification system that a suburban system was built with.
25kV AC substations can be basically pole-mounted - they're basically just a single or 3-phase transformer with a circuit breaker and protection/control relays. A lot less equipment for each substation and fewer substations to boot. That's important when you're electrifying from scratch, which is why Perth, Adelaide and Brisbane chose 25kV AC over 1500V DC.
These days, multi-voltage EMUs and locos are an off-the-shelf commodity. Japan uses multiple electrification schemes and Europe has a whole bunch of different ones too. Mixing the two different electrification schemes together on the same network is quite a lot less of a problem than, say different rail gauges.
The likely outcome is that any country electrification would be done with 25kV AC, because you're building from scratch and can pick whatever system works best. In Melbourne's case, the most appropriate examples of this would be future electrification to Melton, Geelong and/or Wallan. Because you'll probably be using new rollingstock to haul the extra suburban/interurban services, you just need to specify dual voltage EMUs or locos.
For shorter extensions like Mernda or Baxter, sticking with 1500VDC is easier because you're extending existing services and hence want to be able to use existing rollingstock from the get-go.