Starting at 4th level, a druid gains a +4 bonus on saving throws against the spell-like and supernatural abilities of fey. This bonus also applies to spells and effects that target plants, such as blight, entangle, spike growth, and warp wood.
plus, hmm, I don't know, any Plant-type creatures. Let's say a Treant druid.
Since RAW, RNL provides a +4 unnamed bonus on saving throws against spells and effects that target plants, it would, as written, be applied to every single spells and effects used against the Treant Druid because they'll all be used to target the plant creature.
Of course, there's no mistake in what RAI's meant to be but heh, such is RAW :P Never gonna even try getting it past my DM's nose but I thought it would be a nice thought exercise for all of us's.
Your type doesn't change when you WS, unfortunately. A WSed Druid is never a plant, unless they started as one.
Even RAW, it's apparent to me that the spells involved are specifically those that by design target plants, not general-purpose spells that target objects or areas that happen to include plant matter.
E.g., the "target" of Warp Wood is specifically called out as "1 Small wooden object/level, all within a 20-ft. radius," ergo it "targets" plants (specifically wood).