under no circumstance dial *228 from a Thunderbolt--no matter what, no matter why or if any tech in VZW tells you to. it'll render your handset useless.
there are countless threads on people screwing up their phone b/c a vzw tech or sales person told them to dial *228 to activate their phone.
your phone will auto-provision itself, *228 is for CDMA phones on 3g only (i believe). LTE SIM phones don't require this dial code.
as for rooting---i'd NOT root it until you get the new SIM--just in case they try to pull a fast one on you and get you to trade in your current (possibly rooted) handset--which would then create a giant hassle.