If you have at least a small house, you're golden. You don't have to fight other buyers and it is first come-first serve. If you do this, you get a credit of 30% of the minimum value of your current plot towards the purchase. Keep in mind the minimum value is like half the normal plot price. So if your plot cost 3m, min will be like 1.5m, then you only get 30% of the 1.5m as credit towards relocation. If you have decorations in your house during relocation, you have to have enough room in your housing storage for all that to be dumped in, so plan ahead.
Based on what SE said, when they remove the housing demolition freeze, it basically restarts the clock that already existed. So if someone hasn't been in their house in 45 days, it is going to auto-demolish once the freeze goes away. That means server up when 5.4 launches will have the largest number of available plots, because basically everyone who hasn't played the game in 1.5 months/been inside their house will have it poof. You might want to scope out the wards in the game and see if you can figure out what houses might be likely candidates for demo. If you can't figure it out, log out near a main city Aetherite or a mini-aetherite in the housing district and when you log in, check the wards. It will show any empty plots and the price at that point and it will basically be a race to grab plots. Having at least a small will mean you will beat out the plebs that don't even have that.