Permanent TBC server?

Hi everyone! We’re a group of friends planning to get back into playing TBC.
We were wondering if the two TBC servers that are currently online are going to progress to WOTLK at some point, or if they’re meant to stay as permanent TBC realms.
We’re only interested in playing TBC, so we’d like to know before starting.
Thank you!

They are progressive servers that started with base game content and will continue on to Wrath of the Lich King.