TS-Tools ist a good tool for that. Help-File of TS-Tools page 13, You can change existing Assets too.
One Possibility, RwTools, replace Assets. That replaces all!
I have a question. Is there a way to cycle through asset block selection? There has to be an easier way than clicking a hundred times before you get lucky and select the right asset/assetblock.
I dont know, if that works with asset blocks, but you can hold key 2-8 while hovering an asset to cycle through overlapping ones and then select the right one. This cycles through second to eigth nearest 
Yeah too bad, i thought so. But you can still move the assetblocks away one by one, change the trees and hit the undo button. The assetblock returns to its position, but the changed trees stay as they are
Well just like any other asset...click and hold on them and move it aside, to a less populated area, where you can access its setting. Switch the trees and and undo it, that should be it
I think, Holzlaenders Solution is the common and best one.
But in some circumstances you can't do an undo, so if Assets or Assetblocks are in my way I move them aside or up or down with an offset I can easily remember and add/subtract in the x,y,z fields of numbers.
Editing signal functions while it is in a lot of bushes and grass assets is a good example.
Those boundaries should appear while hovering the mouse anywhere over the assetblock and doubleclick inside them should always bring up the flyout, but if that works for you, its okay
