Would like to change conveter/charger

The existing wiring is probably sufficient from a safety standpoint, but you may get better performance (particularly when charging) by upgrading it. Figuring out what size it is would be a good idea, of course. You will get much better charging performance with the new converter regardless.

The exiting fuse panel may need a bit of modification or reconfiguration, as it probably has two electrically separate sections. One is fed by the 30A output of the converter (or the battery, when the converter is not active, via a transfer switch of some sort). The other is connected to the battery at all times and may be listed as "filtered" or something similar. The new converter neither has nor needs this transfer switching mechanism, and all the DC circuits and the battery and the converter output are just wired together in parallel all the time (with the appropriate fuses or circuit breakers for protection). The fuse panel modification basically consists of bridging the two separate sections together somehow.

bestconverter.com has some links to descriptions of converter upgrades that may be helpful to you.

If you have the room, I would leave everything just like it is, except disconnect the ac input power from the old converter, and use it to power a new stand alone PD9245. It will be cheaper (ebay) and no harder to install than a bettery charger. Just plug it in, and connect positive and neg to battery.
If its near the battery, 8 ga wire is more than adequate for the 45 amps.

I would just call PD the have a great tech support and customer service. All of us will give 100 different ways and questions for you. I think if swapping one PD for another PD product they can handle this better. I just did a PD upgrade myself and there customer service was top notch

What size wire is used for the DC wiring now and how long is it? Also, how many coach batteries do you have?

You should probably end up with at least #8 if the length is less than 10', #6 if longer. If you decide to replace the wire, I would go with #4, especially if you have 2 or more batteries.

Afterthought: You are going from a 5 Amp charger (your words) to a 45 Amp charger. The new PD will charge at 45A as long as the battery will accept it, then taper down.

Should just be turn off/disconnect in coming power. Disconnect battery. Remove maybe four screws and Black, hot AC, White, neutral(AC), and Green ground wires.
Remove + DC and - DC wire, these you will want to make larger; say between 8 or 6 AWG.