People are saying "Build the new one first and scavange the old one for parts" and that's like, probably good advice, but if you're anything like me that's just not gonna be realistic. As soon as you start to scavenge, everything will fall apart. I prefer to just trash everything and then rebuild.

Definitely empty your bus, and stockpile way more of the essentials that you think you'll need. So like, double your belt / inserter / assembler / beacon / smelter / pipe / pump / tank / oil&chem stockpiles. Every time I've scrapped my factory I've ended up running out of belts while rebuilding.

Better still, set up a temporary mega mall to churn out hundreds of them, then tear it down after rebuilding your normal mall.

Modules take a long time to handcraft, and depending how developed your current factory is, you may not have a decent stockpile of them. Either way, it's a good idea imo to churn them out while you're rebuilding. If you're not tearing down your smelting/power then those resources are kind of just sitting there, so turn it all into modules. Best case, you won't run out while rebuilding and have to wait to handcraft them. Worst case you end up with a huge stockpile of prod3's that will probably end up useful SOMEWHERE at some point.