I imagine there will be a big uptick in the number of new users but it won’t be a huge wave, not initially. Human nature being what it is, users will switch to the official app, mods will try and muddle through with a more limited toolbox and, slowly, the wheels will fall off as people find that the user experience has been degraded, leading to a fairly steady flow of people moving over (and a decent number moving on as Lemmy likely isn’t ready for primetime just yet). I think what will be key is whether the most experienced moderators and most active posters make the jump - without them the site is a hollow shell as it is largely built on their disproportionate contribution. That might speed up Reddit’s downfall.
The best thing we can do is make their landing as soft as possible.
Make sure there are welcome posts in each instances main communities.
Keep an eye on Reddit for people asking how to make the move.
Keep an eye on the relevant communities here for anyone struggling.