You can replace an Actor in your level with another in your Content Browser. With replacement mesh selected in Content Browser, right click on Actor you want to replace and click Replace Selected Actors With. If you have multiple instances of an Actor you want to replace, you can select them all and do same thing.
It gets more complicated with Foliage, however. foliage you had painted on original Actor will remain in level, but it will not be attached to surface of new Actor. You will have to erase that foliage and repaint, unfortunately.
It’s not likely that this will change, because editing Meshes is not something editor is meant for (though there are some requests in our database for it). Additionally, foliage referencing a face that does not exist on replacement mesh isn’t possible.
There is a workaround, however: you can add LODs to an existing mesh. Right click mesh in your Content Browswer and select ImportLOD. This will let you add an additional LOD level. Then, whichever LOD you don’t want to use anymore, set LOD Distance to an obscenely high number, and it will likely never be loaded. That should work if repainting foliage on a replacement mesh isn’t an option for you, though obviously it’s less clean.
Hope that helps!