They no longer reseed after regionals.
Your seed is your seed throughout in bracket style play through to finals.
In the past, teams were reseeded based on regionals going into nationals.
For example, in 2005, Georgia was ranked 5th with NQS going into the post-season. They went to the South East regionals, had three falls on beam, had to count two falls (Heenan 9.4 and Ericksen 9.225, plus Childs 9.550, dropping Smith's 9.075) and finished with 195.150 to narrowly beat out Denver (17th ranked) who also counted two falls and a 9.5 on beam.
So they dropped from 5th all the way to 12th with that 195.150. This placed them in session 2 the afternoon session with the even ranked teams.
This is a Georgia team that ended up winning the team title and were under ranked all year due to consistency on bars and beam, but hit at the right time.
However, now there is no re-ranking as mentioned.
You could be #18 and the lowest ranked team to make it to Nationals but your ranking at nationals won't change from 18 to 8, it stays 18.